S.Afr.J.Bot., 1994, 60(2): 99 - 107 99 Anatomical adaptations in the leaves of selected

Al ison M. van der Merwe (nee Summerfield),· J.J.A. van der Walt and Elizabeth M. Marais Department of Botany, University of Stellenbosch, Stellenbosch, 7600 Republic of

Received 23 August 1993; revised 6 December 1993

Fynbos experience very harsh conditions during the hot and dry summer months and their leaves are adapt­ ed to reduce the loss of water due to transpiration. The leaves of 46 selected fynbos species of 24 families were examined to determine which anatomical adaptations contribute to the reduced rate of transpiration and subse­ quent reduced water loss. Without exception, all species examined show leaf adaptations typical of xerophytic species. Four typical leaf types are recognized and proposed as models of leaf adaptation: 1. Myrsine type - dorsi ventral or isobilateral leaves; more palisade parenchyma present than spongy parenchyma; tissues contain large amounts of phenolic substances. 2. Meta/asia type - small dorsiventral leaves with involute margins and a single groove in the adaxial surface; mesophyll is usually inverted. 3. Retzia type - dorsi ventral or isobilateral leaves with revolute margins and one or two grooves in the abaxial surface; spongy parenchyma is the main component of the mesophyll. 4. type - small centric or near-centric leaves; little or no spongy parenchy­ ma tissue.

Fynbos plante ondervind uiterste toestande tydens die warm, droa somermaande, en hulle blare is aangepas om waterverlies tydens transpirasie te beperk. Blare van geselekteerde fynbos-spesies uit 24 families is ondersoek am die bydrae van die verskillende anatomiese aanpassings tot verminderde transpirasietempo en gevolglike water­ verlies, vas te stel. AI die spesies wat ondersoek is, vertoon sonder uitsondering, tipiese xerofitiese blaaraanpas­ sings. Vier verskillende blaartipes word beskryf en voorgestel as modelle vir blaaraanpassings: 1. Myrsine-tipe - dorsiventrale- of isobilaterale blare; meer palisadeparenchiem as sponsparenchiem is aanwesig; weefsels bevat groot hoeveelhede fenoliese verbindings. 2. Metalasia-tipe - klein dorsiventrale blare met blaarrande involuut en 'n enkele adaksiale groef; mesofil meestal omgekeerd. 3. Retzia-tipe - dorsiventrale- of isobilaterale blare met blaarrande revoluut en een of twee abaksiale groewe; mesofil bestaan hoofsaaklik uit sponsparenchiem. 4. Spatal­ la-tipe - klein sentriese of byna sentriese blare; min of geen sponsparenchiem .

Keywords: Anatomical adaptations, fynbos, leaf types.

* To whom correspondence should be addressed

Introduction Material and Methods Fynbos vegetation is unique to the and is Twenty-four families occurring in the south-western Cape, readily recognized by the sclerophyllous to microphyllous including all shrubby endemic families, were chosen as represen­ nature of almost all the woody taxa (Kruger 1979). tative of fynbos vegetation. From these families 46 perennial The fynbos biome extends from the Nieuwoudtville escarp­ species were randomly selected and collected in their natural ment to the Gifberg Massif and the Nardouw-Pakhuis-Cedar­ habitat (one species from four localities, two from three, 23 from berg mountains (32°S to 34°S). The inland border runs along two and 20 from one locality, Table 1). The additional species collected from single localities were also included in the study as the base of the sandstone mountain belt of the Witteberg, ' they were valuable for comparisons. Voucher specimens of each Swartberg, Baviaanskloof and Great Winterhoek mountains. species are kept at the Stellenbosch University Herbarium These ranges form an almost continuous chain running west to (STEU). At each locality, ten mature sun leaves were collected east (l8°E to 24°E) and terminating at the Indian Ocean near from more than one of each species. Leaf material was fixed Port Elizabeth (Bond & Goldblatt 1984). in FAA, dehydrated and infiltrated and finally embedded in The climate of the fynbos region is mostly mediterranean or Paraplast, following the tertiary Buthanol method. Transverse semi-mediterranean with a rainfall range of 200 - 3000 mm sections of 5 - 10 fl-m were made through the mid-section of the per annum. The mean annual temperatures throughout the laminae, using a rotary microtome and stained with Alcian Green region are close to 17°C except in the immediate vicinity of Safranin (Joel 1983). the coast where the mean annual temperature range is less than 8°C. Fynbos usually occurs on coarse-grained, infertile, acidic, Results and Discussion sandy soils derived from quartzites and sandstone of the Cape Thick cuticle, waxes and trichomes Supergroup (Cowling & Holmes 1992). In the leaves of all the species examined at least one, if not all, Although fynbos is such a diverse and interesting vegetation, of the above protective coverings were present. very little is known about the anatomy of the leaves of fynbos Although cuticle thickness is not considered to be a very species. The aim of this investigation was to determine which reliable character, in most of the leaves examined the cuticles anatomical adaptations contribute to the reduced rate of trans­ were relatively thick, when compared with the cuticle of a piration and subsequent reduced water loss which enables normal mesophytic leaf. Where a thick cuticle is not present, fynbos plants to survive the hot, dry summer months. Leaves the outer periclinal walls of the epidermal cells are extremely were selected for study as they are the most exposed to aerial thick, as in Roella incurva var. incurva, Aspalarhus cephalores environmental conditions and therefore serve as good indica­ subsp. violacea, Penaea mucronara, Salrera sarcocolla, tors of any anatomical and morphological changes in the plant Thesium ericaefolium, Freylinia lanceolaraand Campylo­ (Mauseth 1988). srachys cernua. A thick cuticle, together with waxes, prevents 100 S .-Afr .Tydslcr .Plantk., 1994, 60(2)

Table 1 List of selected species with collector, collection number and locality

Family/Species Collector Locality

Aizoaceae Galenia africana L. Summerfield 53 Clanwilliam Summerfield 87 Porterville

Asteraceae Brachylaena neriifolia (L.)R.Br. Summerfield 28 Eersterivier Summerfield 79 Jonkershoek, Stellenbosch Eriocephalus africanus L. Summerfield 25 Eersterivier Summerfield 45 Clanwilliam Helichrysum patulum (L.) D.Don Summerfield 50 Clanwilliam Summerfield 61 Jonkershoek, Stellenbosch Heterolepis aliena (L.F.) Druce Summerfield 10 Kogelberg, Betty's Bay Summerfield 86 Gydouw Metalasia densa (Lam.) Karis Summerfield 09 Jan Marais, Stellen bosch Summerfield 23 Kogelberg, Betty's Bay Summerfield 29 Hermanus Metalasia dregeana DC. Summerfield 52 Clanwilliam Metalasia seriphiifolia DC. Summerfield 30 Hermanus

Bruniaceae Berzelia lanuginosa (L.) Brongn. Summerfield 63 Jonkershoek, Stellenbosch Brunia albif10ra E. Phillips Summerfield 12 Kogelberg, Betty's Bay Brunia alopecuroides Thunb. Summerfield 18 Kogelberg, Betty's Bay

Campanulaceae Roella incurva DC. var. incurva Summerfield 16 Kogelberg, Betty'S Bay Summerfield 82 Jonkershoek, Stellenbosch

Ericaceae longifolia Ail. Summerfield 22 Kogelberg, Betty'S Bay Erica plukenetti L. Summerfield 62 Jonkershoek, Stellenbosch Coetzenberg, Stellenbosch

Fabaceae Aspalathus cephalotes Thunb. subsp. violacea Dahl gr. Summerfield 57 Jonkershoek, Stellenbosch Summerfield 74 Coetzenberg, Stellenbosch

Grubbiaceae Grubbia tomentosa (Thunb.) Harms Summerfield 06 Kogelberg, Betty's Bay Summerfield 31 Hermanus

Lobeliaceae Lobelia pinifolia L. Summerfield 32 Hermanus Myricaceae Myrica cordifolia L. Van der Walt 1602 Y zerfontein

Myrsinaceae Myrsine africana L. Summerfield 07 Jan Marais, Stellenbosch Summerfield 26 Eersterivier Summerfield 37 Hermanus Summerfield 43 Clanwilliam

Myrtaceae Metrosideros angustifolia (L.) Smith Summerfield 27 Eersterivier Summerfield 67 Jonkershoek, Stellenbosch

Penaeaceae Penaea mucronata L. Summerfield 14 Kogelberg, Betty'S Bay Summerfield 59 Jonkershoek, Stellenbosch Saltera sarcocolla (L.) Bullock Summerfield 33 Hermanus

Polygalaceae Muraltia heisteria (L.) DC. Summerfield 38 Hermanus Summerfield 56 Jonkershoek, Stellenbosch Diastellafraterna Rourke Summerfield 15 Kogelberg, Betty's Bay repens (L.) L. Summerfield 08 Jan Marais, Stellenbosch Summerfield 72 Coetzenberg, Stell en bosch kraussii Meisn. Summerfield 68 Jonkershoek, Stellenbosch Summerfield 92 Porterville Spatalla mollis R. Bf. Summerfield 17 Kogelberg, Betty's Bay S.Afr.J.Bot., 1994, 60(2) 101

Table 1 Continued

Family/Species Collector Locality

Retziaceae Relzia capensis Thunb. Summerfield 11 Kogelberg, Betty's Bay

Rhamnaceae Phylica buxifolia L. Summerfield 34 Hermanus Summerfield 90 Citrusdal Phylica cryptandroides Sond. Summerfield 46 Oanwilliam Phylica lasiocarpa Sond. Summerfield 21 Kogelberg, Betty' s Bay Phylica spicala L.f. Summerfield 55 Jonkershoek, Stellenbosch Summerfield 91 Citrusdal Phylica slipularis L. Summerfield 58 Jonkershoek, Stellenbosch Summerfield 89 Gydouw

Roridulaceae Roridula gorgonias Planch. Summerfield 13 Kogelberg, Betty's Bay

Rosaceae Clifforlia ruscifolia L. Summerfield 02 Jan Marais, Stellenbosch Summerfield 44 Oanwilliam Summerfield 66 Jonkershoek, Stellenbosch

Rutaceae Agalhosma bifida (Jacq.) Bartl. & Wendl. Summerfield 41 Hermanus Diosma hirsula L. Summerfield 20 Kogelberg, Betty's Bay Summerfield 95 Jonkershoek, Stellenbosch Diosma subulata Wendl. Summerfield 42 Hermanus

Sapindaceae Dodonaea angustifolia L.f. Summerfield 49 Oanwilliam Summerfield 69 Jan Marais, Stellenbosch

Santalaceae Thesium ericaefolium L. Summerfield 36 Hermanus Summerfield 94 Jonkershoek, Stellenbosch

Scrophulariaceae Freylinia lanceolala (LJ.) G. Don Summerfield 24 Eersterivier Summerfield 60 lonkershoek, Stellenbosch

Stilbaceae Campyloslachys cernua eL.f.) Knuth Summerfield 19 Kogelberg, Betty's Bay

Thymelaeaceae Passerina glomerala Thunb. Summerfield 47 Oanwilliam Summerfield 88 Gydouw Passerina vulgaris Thoday Summerfield 40 Hermanus Summerfield 65 Jonkershoek, Stellenbosch Strulhiola leplamha Bolus Summerfield 51 Oanwilliam Struthiola myrsiniles Lam. Summerfield 35 Hermanus Summerfield 83 Jonkershoek, Stellenbosch

cuticular transpiration and reflects excessive sunlight (Mauseth africana dendritic hairs are also present. On the leaves of 1988). Roridula gorgonias extremely large multicellular hairs occur Waxes occur regularly, especially in the vicinity of the adaxially and abaxially. Trichomes confined to the grooves stomata on the more exposed leaf surface, for example in occur in Heterolepis aliena, Metalasia densa, M. dregeana, M. Myrsine africana, Myrica cordifolia, Metrosideros angusti­ seriphiifolia, Erica longifolia, E. plukenetii, Grubbia tomen­ folia, , Diastellafraterna and Passerina vulgaris. tosa, Campylostachys cernua, Passerina glomerata, P. vulga­ Non-glandular or glandular hairs occur in many of the ris, Phylica buxifolia, P. cryptandroides, P. lasiocarpa, P. species examined. Trichomes are often confined to a groove in spicata and P. stipularis. This possibly restricts the amount of either the adaxial or the abaxial leaf surface, as in Heterolepis water loss through the trichomes. As the water evaporates from aliena, Metalasia densa, M. dregeana, M. seriphiifolia, Erica the trichomes, the air in the groove becomes saturated with longifolia, E. plukenetii, Grubbia lomentosa, Campyloslachys water vapour and consequently suppresses further water loss. cernua, Passerina glomerata, P. vulgaris as well as the repre­ In Roridula gorgonias, there are no grooves in the leaf surface sentatives of the Rhamnaceae. According to De Lange (1992), and it may be significant that this species occurs in shaded, the hairs probably decrease air movement over the abaxial moist areas and therefore may not need the added protection epidermis as well as reducing solar input; both strategies that the other species require. Another function of trichomes diminish the rate of transpiration. Peltate glandular hairs occur may be protection against excessive exposure to sunlight, as in Myrsine africana and Myrica cordifolia and in Myrsine Esau (1977) states that when non-glandular hairs die and 102 S.-Afr.Tydskr.Plantk., 1994, 60(2)

Thesium ericaefolium and Campylostachys cernua, all epider­ mal cell walls are thickened. Phenolic substances occur in the epidermal cells of Brunia albiflora, Spatalla mollis, Retzia capensis, Cliffortia ruscifolia, Diosma subulata, Passerina glomerata, P. vulgaris and Struthiola leptantha. Mucilage was found in the epidermal cells or within the cell walls in the leaves of Erica longifolia, E. plukenetii, Aspalathus cephalotes subsp. violacea (Figure 2), Struthiola myrsinites and others. According to Jordaan & Theunissen (1992), phenolic depo­ sits often occur in the mucilage of the epidermal layers of most xerophytic plants, and both Lyshede (1977) and Fahn (1990) suggested that mucilage may be involved in the water economy of the leaves of xerophytes. According to Mauseth (1988) mucilages are secretions containing carbohydrates and have an extremely high water content. As mucilage is hydrophilic and attracts water to itself, it could well act as a very successful water storage mechanism within the epidermal walls. When conditions become unfavourable the mucilage within the epidermal cells may dry out and release the stored water to the cells. Glands occur as small dots on the leaf surfaces of Agathos­ rna bifida, Diosma hirsuta and Diosrna subulata. According to Fahn (1990), several authors have suggested that the vapour of ethereal oil may lower the rates of evaporation and Figure 1 Transverse section through leaf of Agatfwsma bifida. transpira ti on. m: multiseriate epidermis. Scale bar = 50 f.lm. Stomata dehydrate, their walls become refractive and scatter light to All the leaves examined have numerous stomata occurring protect the leaf from excessive sunlight. mostly abaxially, often on both sides and more rarely only adaxially. The advantage to the plant of many smaller stomata is that the humid zones formed above each stoma overlap to Epidermal cells form a single large humid zone, from which less water is lost The epidermis is usually present as a single layer, although a to the atmosphere than would be from many separate humid multiseriate epidermis may occur in some species such as zones (Mauscth 1988). The stomata are frequently protected by Erica longifolia, Aspalathus cephalotes subsp. violacea, Aga­ being sunken, as in Eriocephalus africanus, Aspalathus cepha­ tfwsma bifida (Figure I), Diosma subulata and Struthiola lotes subsp. violacea and Freylinia lanceolata, or they may be leptantha. The epidermal cells are relatively small in most of surrounded by flanges of cuticle like those of Brachylaena the species examined. With the exception of Galenia africana, neriifolia, Berzelia lanuginosa, Brunia albiflora, B. alopecu­ Grubbia tomentosa, Retzia capensis and Roridula gorgonias, roides, Aspalathus cephalotes subsp. violacea (Figure 3), the outer periclinal cell walls are thickened. In certain species, for example, Roella incurva vaT. incurva, Lobelia pinifolia,

]

Figure 2 Aspalathus cephalotes subsp. violacea transverse Figure 3 Aspalathus cephalotes subsp. violacea transverse section through leaf showing mucous present within epidermal section through stoma to illustrate cuticular flanges. c = cuticular cells. m = mucous. Scale bar = 100 f.lm. flanges. S.Afr.1.Bot., 1994,60(2) 103

Lobelia pinifolia and others. The leaves of fynbos plants are very often adpressed to the stem or densely imbricate or even both. The surface of the leaf which is held against the stem is therefore relatively protected from harsh sunlight and wind and the stomata are very often more abundant on this protected surface. The oblique exposure of the surface of the adpressed leaves to the sun's radiation should result in lower leaf temperatures and less exposure of the trichome-free adaxial surface than would be the case with ref1exed leaves (De Lange 1992). Another adaptation is that the stomata are confined to a groove in this surface, as in lleterolepis aliena, Metalasia densa, M. dregeana, M. seriphiifolia, Erica longifolia, E. plukenetii, Grubbia tomentosa, Myrica cordifolia, Campylosta­ chys cernua, Passerina glomerata, P. vulgaris, and the repre­ sentatives of the Rhamnaceae. Maximov (1931) states that the antagonism between the processes of photosynthesis and transpiration have a deep influence on leaf structure because the same stomatal openings that serve for the diffusion of carbon dioxide into the leaf, also serve for the diffusion of water vapour out of the intercellular spaces into the surrounding atmosphere. For the retention of water, plants must pay with hunger in carbon and for supply of carbon they have to pay with increased water loss (Maximov 1931). This problem is overcome to a certain degree when the stomata are confined to a groove in the leaf surface, as it seems that the stomata are able to assimilate a maximum amount of carbon dioxide with minimum water loss. As is the case with Figure 4 Roella incurva var. incur va transverse section through the trichomes which are situated in a groove, the surrounding leaf showing the presence of thick-walled sclerenchymatic fibres atmosphere in the groove becomes saturated with water beneath the epidermis. f = sclerenchymatic fibres. vapour. As there is reduced air movement in the groove to remove the water vapour arising from the stomata, less water may be lost through transpiration. This would appear to be a intensity which is directly proportional to cell diameter. highly successful adaptation to reduce the loss of water Therefore, the smaller the cell diameter, the less intense the through transpiration without influencing the rate of photosyn­ exposure to sunlight. thesis, as it appears that the stomata are seldom forced to close. The development of additional palisade is advantageous for the plant because water transport towards the epidermis is Hypodermis much higher through palisade than through spongy parenchyma A hypodermis occurs in the leaves of many of the species and densely packed palisade cells also increase water transport examined. This is usually a single layer of thin-walled paren­ as water is able to move in more than one plane (Mauseth chymatic cells and may be confined to the adaxial, more 1988). exposed leaf surface, as in Roridula gorgonias, or may occur abaxially in the region of the midrib, as in lleterolepis aliena. A hypodermis may even be present both adaxially and abaxi­ ally as in Brachylaena neriifolia, Agathosma bifida, Diosma subulata, Freylinia lanceolata and Struthiola myrsinites. In Roella incurva var. incurva, a hypodermis consisting of three layers of small, thick-walled sclerenchymatic fibres occurs beneath the adaxial epidermis as well as beneath the abaxial epidermis in the region of the midrib and at the leaf margin (Figure 4). In some species the cells of the hypodermis contain phenolic substances or mucilage. The main function of the hypodermis seems to be that of excluding excessive sunlight (Mauseth 1988). Where phenolic deposits are present there may be an added function of water storage, as phenolic sub­ stances are thought to be hydrophilic and therefore attract and bind water to themselves. This water may then be re-released to the plant when conditions become unfavourable (Jordaan & Theunissen 1992).

Palisade and spongy parenchyma In virtually all the species examined there is an increase in the amount of palisade parenchyma at the expense of the develop­ ment of spongy parenchyma. In Agathosma bifida, Jraterna and Struthiola myrsinites (Figure 5) there is very little spongy parenchyma and in Serruria kraussii and Spatalla 1-----1 mollis there is no spongy parenchyma at all. The palisade cells 50um in almost all species are narrow, elongated and densely packed. According to Bokhari & Wendelbo (1985), narrow palisade Figure 5 Struthiola myrsmltes transverse section through leaf cells offer more protection to chloroplasts against intense showing the minimal amount of spongy parenchyma. p = palisade illumination, as palisade tissues are subjected to a light parenchyma; s = spongy parenchyma. 104 S.-Afr.Tydskr.Plantk., 1994, 60(2)

Of the species studied, 19 have dorsi ventral leaves, 20 have isobilateral leaves with palisade parenchyma beneath each epidermis and seven species have centric or near centric leaves. In all three Metalasia species examined, as well' as in Passerina glome rata and P. vulgaris, the mesophyll is invert­ ed, The palisade parenchyma occurs abaxially with the spongy parenchyma adaxially. In the Metalasia species, however, the leaves are twisted through 1800 resulting in the positioning of the palisade beneath the upper (abaxial) epidermis, It is quite clear that this is the abaxial epidermis because of the position of the xylem (Figure 6). In all species, the spongy parenchyma cells are loosely arranged, but have surface projections which extend from one spongy cell to another. This is a very impor­ tant adaptation because intercellular air spaces are necessary for the diffusion of carbon dioxide into and out of the cells. However, too many intercellular air spaces can hinder the movement of water within the tissues. The surface projections serve to increase the cell surface area, thus increasing the total surface area of the tissue while at the same time preserving the intercellular air spaces necessary for the normal diffusion of carbon dioxide within the leaf.

Vascular tissues Most of the species examined are highly vascularized. The vascular bundles are enclosed in bundle sheaths, which are most often parenchymatic in nature, although sclerenchymatic bundle sheaths may occur. Bundle sheath extensions are rela­ tively rare but are present in certain species, for example, Figure 7 Brachylaena neriifolia transverse section through leaf showing bundle sheath extensions of a secondary vascular bundle. Brachylaena neriljolia, Helichrysum patulum, He tero lep is b bundle sheath extensions; d adaxial epidermis; h hypo­ aliena, Myrica cordifolia, Phylica buxifolia, P. cryptandroides, = = = dermis. P. lasiocarpa, P. spicata and p, stipularis. These bundle sheath extensions may be parenchymatic (Figure 7) or collen­ chymatic. According to Esau (1977), bundle sheath extensions This tissue also plays a role in the protection and support of the playa major role in the distribution of water away from the vascular bundles. xylem and out to the mesophyll; they also play a mechanical role in support and protection of the vascular bundles. In Fibres and sclereids certain species the vascular bundles of the midrib are accompa­ Both fibres and sclereids are common in the leaves of the nied by arcs of sclerenchyma as visible in Cliffortia ruscifolia, Passer ina glomerata, P. vulgaris and Struthiola leptantha.

Figure 6 Meta/asia densa transverse section through leaf Figure 8 Penaea mucronata transverse section through meso­ showing the inverted mesophyll. a = adaxial epidermis; d = abax­ phyll of leaf showing the presence of asterosclereids. a = astero­ ial epidermis; p = palisade parenchyma; s = spongy parenchyma. sclereid; p = palisade parenchyma; s = spongy parenchyma. S.Afr.J.Bot.,1994,60(2) 105

species examined. They often occur within the vascular bun­ to Jordaan & Theunissen (1992), the amount of phenolic dles or within close proximity to them and are thought to play deposits observed microscopically can be related to the quanti­ a role in protecting the tissues against mechanical injury on tatively determined tannin concentration and that the concen­ dessication. The leaves of many of the species examined are tration of tanniniferous substances increases when drought made up of a large proportion of thick-walled sclerenchyma, conditions intensify. They also stated that phenolic deposits which maintains structural rigidity and prevents wilting. may be part of an anti-herbivory defence mechanism or the Asterosclereids occur scattered throughout the mesophyll in increase in phenolic substances may also be the result of a Penaea mucronata and Saltera sarcocol/a, both members of disrupted metabolism caused by the high radiation levels of the Penaeaceae (Figure 8). In most species examined fibre caps arid regions. Enhanced photosynthesis occurs under conditions occur at either one or both poles of the vascular bundles. In of high light intensity and this results in the formation of large CllfJortia ruscifolia, Passerina glomerata, P. vulgaris and amounts of carbohydrates, which leads to an increased ratio of Struthiola leptantha, strands of sclerenchymatic fibres run carbon to nitrogen and the production of more carbon-based parallel to the leaf margins in the mesophyll. These fibre compounds (Jordaan & Theunissen 1992). strands seem to offer mechanical support to the leaf margins and protection against tearing by wind. Research has shown Conclusions that the amount of sclereidal and fibrous elements increases Fynbos species experience very harsh conditions during the during times of drought, but neither the reason for the increase, summer months as there is little or no available water. As a nor the precise function of the elements is clear (Shields 1950). result, perennial fynbos species with persistent above-ground parts have had to develop anatomical adaptations to the dry Phenolic deposits conditions in order to survive. The leaves of the species Esau (1977) stated that no tissue is entirely devoid of tannins selected for this study vary in size and form but all possess and in all species examined, except for Galenia aJricana, certain anatomical adaptations or combinations thereof. These phenolic deposits occur. Phenolic deposits were found in the adaptations allow fynbos species to exercise maximum photo­ epidermal cells, in the cells of the palisade and spongy paren­ synthesis with minimum water loss through transpiration, thus chyma, the bundle sheath cells and the cells of the bundle enabling them to function very effectively in the arid summer sheath extensions as well as in the parenchymatic tissue of the months. In all the species examined there are four combina­ phloem and xylem. Phenolic deposits are not present in all tions of adaptations which commonly occur and these four these tissues in every species, but in all species many of the different leaf types have been proposed as models of leaf tissues mentioned do contain phenolic substances. According adaptation in fynbos species (Figure 9).

phloem = sclerenchyma = • xylem = 11111111

Figure 9 A graphic representation of the four leaf types. (a) Myrsine leaf type; (b) Metalasia leaf type; (c) Retzia leaf type; (d) Spatalla leaf type. 106 S.-Afr.Tydskr.Plantk., 1994, 60(2)

1. Myrsine leaf type (Figure 9a) Table 3 List of species within each The species which fall into this category of leaf type (Table 2) family with the Meta/asia leaf type have either dorsi ventral or isobilateral leaves without any major grooves in either surface, although small grooves or Family Species crypts may occur. The cuticle is usually thick. The outer Asteraceae M etalasia densa periclinal walls of the epidermal cells are thickened and in M etalasia dregeana some cases all the epidermal cell walls may be thickened. The Metalasia seriphiifolia epidermal cells often contain ergastic substances as is charac­ Rutaceae Diosma subulata terized by Myrsine africana. Trichomes occur both adaxially Thymelaeaceae Passerina glomerata and abaxiallyor may be confined to the abaxial surface. Peltate Passerina vulgaris glandular hairs and dendritic hairs may also be present. A hypodermis, with or without phenolic substances, commonly occurs adaxially or may be confined to the abaxial midrib region. Leaves are usually hypostomatic, although in some walls) forming the upper leaf surface. The cuticle of the species they may be amphistomatic. Stomata occurring adaxial­ adaxial surface is thin and the epidermal cells of the two ly are protected by cuticular flanges, they may also be confined surfaces differ. The adaxial epidermal cells are small and to small grooves in either leaf surface. irregularly shaped in comparison with the larger abaxial Palisade parenchyma occurs adaxially and spongy parenchy­ epidermal cells with their thickened outer periclinal walls. The ma abaxially, or the palisade parenchyma may occur beneath abaxial epidermal cells may contain phenolic compounds. The both epidermises with the spongy parenchYfQa in the centre. In leaves are generally epistomatic with the stomata being con­ both cases there is a large amount of palisade parenchyma and fined to a groove in the adaxial surface. Unicellular hairs are the cells of the spongy parenchyma extend surface projections confined to the adaxial groove. The inverted mesophyll com­ towards one another thus increasing the surface area of the prises one to two layers of densely packed palisade parenchy­ spongy tissue. Fibres occur within the vascular bundles and all ma cells abaxially, and spongy parenchyma adaxially. The vascular bundles are enclosed in a parenchymatic or scleren­ spongy parenchyma consists of irregularly-shaped, loosely chymatic bundle sheath. The vascular tissues of most of the arranged cells with extended surface projections from one to species exhibiting the Myrsine leaf type also contain a large another. Each vascular bundle is enclosed in a parenchymatic amount of phenolic compounds. Bundle sheath extensions to­ bundle sheath, the cells of which may contain phenolic depo­ wards both epidermises may occur. Sclerenchymatous elements sits. Fibres often occur within the vascular bundles. Columns often occur in close proximity to the vascular bundles and are of sclerenchymatic fibres may be found between the vascular enclosed with the vascular bundle in a parenchymatic bundle bundles and the abaxial epidermis; these strands may also sheath, the cells of which contain phenolic deposits. Thick­ extend laterally beneath the abaxial epidermis. Diosma subula­ walled asterosclereids may be found scattered throughout the ta has been placed in the M etalasia leaf type because of the mesophyll and directly beneath the epidermis. Secretory canals small groove in the adaxial surface, although this species containing mucilage sometimes occur in the leaf mesophyll. seems to be a transition form between the Myrsine and Metalasia leaf types. Diosma subulata has isobilateral, hypo­ 2. Metawsia leaf type (Figure 9b) stomatic leaves with a possible multiple epidermis, the inner The leaves of species belonging to this type (Table 3) are layer of which is filled with a mucilaginous substance. The characterized by dorsiventral, involute leaves where the leaf mesophyll is composed of a single layer of palisade paren­ margins curl upwards forming an adaxial groove. The meso­ chyma beneath each epidermis with loosely arranged spongy ph¥ll is inverted with the palisade parenchyma occurring abaxi­ parenchyma forming the central mesophyll. The cells of the ally and the spongy parenchyma adaxially. In all the Metalasia spongy parenchyma exhibit the characteristic surface projec­ species examined the leaves are twisted through 1800 with the tions which extend from one cell to another. Secretory canals abaxial surface (with a thicker cuticle and thickened epidermal occur in the spongy parenchyma on the abaxial side of the leaf. The vascular bundles are enveloped in parenchymatic bundle sheaths, the cells of which contain phenolic deposits. Phenolic Table 2 List of species within each deposits also occur in the palisade parenchyma tissue. family with the Myrsine leaf type 3. Retzia leaf type (Figure 9c) Family Species Species in this group (Table 4) have dorsi ventral or isobilateral leaves. They are all, however, characterized by an abaxial Aizoaceae Galenia africana groove each side of the midrib (with the exception of the two Asteraceae Brachylaena neriifolia Erica species which have a single abaxial groove) formed by Helichrysum patulum the revolute margins of the leaf as they curl under. The cuticle Lobeliaceae Lobelia pinifolia is relatively thick except opposite the abaxial grooves. Where a Myrsinaceae Myrsine africana thin cuticle occurs, the outer periclinal walls of the epidermal Myricaceae Myrica cordifolia cells are greatly thickened. The phenolic compound-containing, Myrtaceae Metrosideros angustifolia ordinary epidermal cells are large with thickened outer peri­ Penaeaceae Penaea mucronata c1inal cell walls. The epidermal cells of the abaxial grooves are Saltera sarcocolla somewhat smaller than those occurring adaxially. Stomata are PolygaJaceae Muraltia heisteria mostly confined to the abaxial grooves but may also occur Proteaceae Diastella Jraterna adaxially, in which case they are protected by cuticular flanges. Pro tea repens In amphistomatic leaves the stomata are more abundant abaxi­ Roridulaccae Roridula gorgonias Rosaceae Cliffortia ruscifolia ally. Unicellular hairs are confined to the abaxial grooves. The Rutaceae Agathosma bifida mesophyll comprises one to two layers of densely packed pali­ Sapindaceae Dodonaea angustifolia sade parenchyma which may be confined to the adaxial side of Scrophulariaceae Freylinia lanceolata the leaf or may occur beneath both the adaxial and the abaxial Thymelaeaceae Struthiola leptantha epidermis. Palisade parenchyma is always absent opposite the Struthiola myrsinites abaxial grooves. The palisade parenchyma contains phenolic substances. The spongy parenchyma is loosely arranged with S.Afr.J.Bot., 1994, 60(2) 107

Table 4 List of species within each The four leaf models which have been proposed are the Jamily with the Retzia leaf type outcome of the successful combinations of various adaptations which enable leaves, that appear very different in structure, to Family Species be adapted to the same environment. The leaves of each spe­ Asteraceae H eterolepis aliena cies, in every family examined, can be grouped into one of the Ericaeae Erica longifolia four leaf types, with transitional types also occurring. Erica plukenetii It seems that the anatomical adaptations in the leaves of all Grubbiaceae Grubbia tomentosa the species studied are possibly aimed at reducing transpiration Retziaceae Retzia capensis while promoting maximum photosynthesis. Speculation still Rhamnaceae Phylica buxifolia exists as to why these adaptations occur in fynbos plants and Phylica cryptandroides what the stimulus was for their initial development. Shields Phylica lasiocarpa (1950) has related xeromorphic structure to low nitrogen Phylica spicata content and has attributed sclerophylly to low levels of Phylica stipularis phosphate. Esau (1977), however, states that the availability of Stilbaceae Campylostachys cemua water is the most important factor affecting the form and structure of plants. Stock et ai. (1992) have found that water appears to act as a selective pressure in the fynbos region only on nutrient-rich soils. According to this, as most fynbos surface projections extending from one cell to another. The vegetation occurs in nutrient-poor soils, water cannot be vascular bundles include many fibres and are enclosed in considered to be a selective pressure in the fynbos region. parenchymatic bundle sheaths. A region of collenchymatic Although there is still much controversy as to which factor tissue may occur between the main vascular bundle and abaxial stimulates the xeromorphic adaptations found in fynbos plants, epidermis. the fact that the leaves exhibit xeromorphic adaptations, which enable the plants to reduce the amount of water lost through 4. Spataila leaf type (Figure 9d) the leaves during transpiration, cannot be disputed. The leaves of these species (Table 5) are imbricate or The adaptations in the leaves of the species which were adpressed to the stem and centric or near-centric in shape (the examined are of little taxonomic value because of the conver­ leaf of Serruria kraussii shows a small remnant of an adaxial gence of the different species towards the four leaf types. groove) and are rich in either phenolics or mucilage. The cuticle is usually thin and in almost all the species the ordinary Acknowledgements epidermal cells are interspersed with larger, more rounded The results of this study formed part of an M.Sc. thesis in the cells. All the epidermal cell walls may be thickened, or cell Botany department of the University of Stellenbosch under the wall thickenings may be restricted to the outer periclinal walls. guidance of Prof. J.J.A. van der Walt and Miss E.M. Marais The epidermal cells usually contain phenolic substances, and sincere appreciation is expressed for their assistance. although they may sometimes contain mucilage. Stomata are level with the leaf surface and are protected by cuticular References flanges, or may be slightly sunken. A hypodermis is common BOKHARI, M.H. & WENDELBO, P. 1985. Anatomy of Dionysia. II. beneath the adaxial epidermis as well as beneath the abaxial Xeromorphic features. Notes RBG Edinb. 42(2): 327 - 345. epidermis in the region of the midrib. A large amount of BOND, P. & GOLOB LA IT, P. 1984. Plants of the Cape flora. JI S. palisade parenchyma occurs beneath the epidermis along the Afr. BOI. Suppl. Vol. 13. entire leaf circumference. The cells are densely packed and COWLING, R.M. & HOLMES, P.M. 1992. Aora and vegetation. In: contain phenolic compounds. Very little or no spongy paren­ The ecology of fynbos, nutrients, fire and diversity, ed. R. chyma is present but if present the cells are rounded and Cowling. Oxford University Press. densely packed. The vascular system often only consists of the DE LANGE, J.H. 1992. Autecology and embryology of Audouinia single main vascular bundle or in some cases this main bundle capilala (Bruniaceae). A threatened species in the Cape Aoristic is flanked on each side by a secondary bundle. The vascular Region. Ph.D. thesis, University of Stellenbosch. tissues contain many fibres and each vascular bundle is enclos­ ESAU, K. 1977. Anatomy of seed plants, 2nd edn. John Wiley and ed in a parenchymatic bundle sheath, the cells of which contain Sons, New York. phenolic substances. Extraxylary fibres often form a phloem FAlIN, A. 1990. Plant anatomy, 4th edn. Pergamon Press, Oxford. fibre cap in the main vascular bundle or fibre caps may be JOEL, D.M. 1983. AGS (Alcian Green Safranin). A simple differential staining action of plant material for the light present at both poles of the vascular bundle. microscope. Proc. R.M.S. 18: 149. JORDAAN, A. & THEUNISSEN, J.D. 1992. Phenolic deposits and tannin in the leaves of five xerophytic species from Southem Table 5 List of species within each family with the Africa Bot. Bull. Academia Sinica 33: 55 - 61. Spatalla leaf type KRUGER, FJ. 1979. Fynbos ecology: a preliminary synthesis. South African National Scientific Programmes, report no. 40. J. Day, Family Species W.R. Siegfried, G.N. Louw & M.L. Jarman, eds. Asteraceae Eriocephalus africanus LYSHEDE, O.B. 1977. Studies on the mucilaginous cells in the leaf Bruniaceae Berzelia lanuginosa of Sparlocytisus jilipes W.B . Planta 133: 255 - 260. Brunia albij10ra MAUSETH, J.D. 1988. Plant anatomy. Benjamin Cummings Publish­ Brunia alopecuroides ing Co;. Inc. Menlo Park, California. Campanulaceae Roella incurva var. incurva MAXIMOV, N.A. 1931. The physiological significance of the xero­ Fabaceae Aspalathus cephaloles subsp. violacea morphic structure of plants. 1. Ecol. 19: 272 - 282. Proteaceae Serruria kraussii SHIELDS, L.M. 1950. Leaf xeromorphy as related to physiological Spalalia moliis and structural influences. Bot. Rev. 16: 399 - 477. Rutaceae Diosma hirsuta STOCK, W.O., V AN DER HEYDEN, F. & LEWIS, O.A.M. 1992. Santalaceae Thesium ericaefolium Plant structure and function. In: The ecology of fynbos, nutrients, fire and diversity, R. Cowling, ed. Oxford University Press.