S.Afr.J.Bot., 1992, 58(6): 461 - 468 461 Fruit polymorphism in ephemeral species of Namaqualand. V. Intramorphic competition among cultivated from dimorphic diaspores

Karen Beneke,* Margaretha W. van Rooyent and G.K. Theront 'Grassland Research Centre, Private Bag X05, Lynn East, 0039 Republic of South Africa tOepartment of Botany, University of Pretoria, Pretoria, 0002 Republic of South Africa

Received 27 February 1992; revised 22 June 1992

Dimorphotheca sinuata and fastuosa, two polymorphic annual species of Namaqualand, Republic of South Africa, were studied to determine the influence of density on the production of plants cultivated from the disc and ray, and black and brown diaspores, respectively. There was no significant difference between the biomass production of plants cultivated from the different diaspore types. Intramorphic competition was stronger in the disc and black plants than in the ray and brown plants, respectively. In both species biomass allocation to parts did not differ between the polymorphs.

Dimorphotheca sinuata en , twee polimorfiese eenJange spesies van Namakwaland, Republiek van Suid-Afrika, is bestudeer om die invloed van digtheid op die produksie van plante, wat onder­ skeidelik vanaf buis- en lint-, en swart en bruin diaspore gekweek is, te ondersoek. Oaar was geen betekenis­ volle verskille tussen biomassaproduksie van plante gekweek vanaf die verskillende diaspoortipes nie. Intra­ morfiese kompetisie was sterker in buis- en swart plante as in lint- en bruin plante, onderskeidelik. In beide spesies was daar geen verskil in die biomassatoewysing aan plantdele tussen die polimorfe nie.

Keywords: Arctotis fastuosa, arid environment, density, Dimorphotheca sinuata, Namaqualand.

'To whom correspondence should be addressed.

Introduction conditions arise: escape in time or in space (Maun & Payne Namaqualand is an arid region with a winter rainfall and 1989). supports an abundant flora of annual plants (Le Roux & Polymorphism occurs in 8.5% of the ephemeral species of Schelpe 1988). These plants complete their life-cycles in Namaqualand. Two of these ephemeral species, which pro­ short periods following substantial rainfall (Beatley 1974; duce mass displays, Dimorphotheca sinuata DC. and Mulroy & Rundel 1977) and survive unfavourable warm, ArctotisJastuosa Jacq. were chosen for an intensive study to dry periods as seeds. compare the success of the different morphs. In the case of According to Firbank and Watkinson (1985), one of the D. sinuata the morphs differed in shape, whereas in the case major factors influencing the growth and survival of in­ of A. Jasluosa the difference between the morphs was only dividual plants is competition from neighbours. The yield of in colour. a plant species in monoculture depends on density and the The differences and similarities in morphology and germination of the dimorphic diaspore types of these species length of the growing period. Shortly after sowing there is a were described by Beneke (1991) and Beneke et al. direct proportionality between total weight per plant per unit (1992a-c). Differences were also found in various growth area and density. With time, competition reduces the mean parameters between plants cultivated from the dimorphic plant weight at higher densities to give near constant yield diaspores of D. sinuata (Beneke 1991, Beneke et al. 1992d). over an increasingly wide range of high densities (Baeumer To determine whether a particular morph would have an & De Wit 1968). advantage over the other under certain environmental condi­ For each species there is a maximum density that can be tions, the sensitivity of the different morphs to various forms obtained, and densities beyond this level cannot be realized of stress, e.g. density, nutrient and moisture stress, was (Harper 1967). The direct consequences of density stress on examined. The aim of this study was therefore to determine plant populations are: (i) to elicit a plastic response from (i) the effect of intramorphic competition on plants culti­ individuals as they adjust to share limiting resources, (ii) to vated from different diaspore types of two polymorphic increase mortality, and (iii) to exaggerate differentials and species, (ii) at what density the plateau yield is reached, and encourage a hierarchy within the population. Density stress (iii) whether there are any differences in the yield of plants intensifies the expression of small differences (genetic or cultivated from the different diaspore types in competition. environmental) between individuals (Harper 1967). Fruit polymorphism, the production of two or more Materials and Methods morphologically distinct types of diaspores by an individual Diaspores used in this study were collected in the Goegap plant, typically occurs in the 's annual species Nature Reserve in August 1989. The reserve is situated in occurring in arid habitats (Bachmann et al. 1984; Tanowitz the Namaqualand Broken Veld (Acocks 1988), in the north­ et al. 1987). The production of polymorphic diaspores en­ western corner of the Republic of South Africa. The climate ables species to adopt two strategies when unsuitable of the area is characterized by warm, dry summers and cold, 462 S.-Afr.Tydskr.Plantk., 1992, 58(6)

moist winters. The average annual temperature is 17.rc Table 1 The influence of density on the area of and the average annual rainfall is 162 mm, falling mainly in Dimorphotheca sinuata and Arctotis fastuosa. Statistically winter (Weather Bureau 1988). homogenous groups (p = 0.05) are indicated under each The embryos of the winged-disc and unwinged-ray dia­ group. Data are means ± standard deviations spores of Dimorphotheca sinuata DC. as well as the em­ bryos of the black and brown diaspores of Arctotis jastuosa Density Jacq. were excised (to increase germination) and germinated (plants per pot) Per plant Per pot in Petri dishes. After one week the seedlings were planted Dimorphotheca sinuata out of doors at the University of Pretoria into 1.25 drn 3 pots filled with 1.638 kg quartz sand with a particle size of 0.8 - Disc plants 1.6 mm. The plants were cultivated at densities of one, two, 1 368.39 ± 114.73 368.39 ± 114.73 four and eight plants per pot. All plants were watered daily 2 262.39 ± 100.16 524.76 ± 164.15 4 190.31 ± 71.14 761.23 ± 106.41 to field capacity with tap water and received Amon & 8 95.88 ± 37.73 767.04 ± 117.04 Hoagland's complete nutrient solution (Hewitt 1962) once a week. 8 4 2 1 1 2 4 8 After five months, five to six replicates for each density Ray plants were harvested, and the following values were determined 1 328.84 ± 93.80 328.84 ± 93.80 per plant and per pot: (a) leaf area (cm2); (b) number of 2 268.40 ± 95.20 536.81 ± 96.38 buds; (c) number of open ; 4 157.43 ± 84.65 629.71 ± 53.96 (d) number of fruitheads; (e) number of empty fruitheads; 8 82.03 ± 55.69 656.25 ± 49.09 dry and (t) total number of inflorescences (b+c+d+e). The 8 4 2 1 124 8 mass (g) values for the entire plant as well as those of the organs were also determined on a per plant and per pot Arctotis fastuosa basis. Black plants Leaf areas were determined with a LiCor LI 3100 leaf 1 735.13 ± 218.77 735.13 ± 218.77 area meter. The roots and above-ground parts were sepa­ 2 427.61 ± 219.21 855.23 ± 281.23 rated at ground level. To determine dry mass the plant parts 4 328.19 ± 204.27 1029.73 ± 531.49 were dried at 60°C for one week, to a constant mass. 8 167.60 ± 100.15 1340.83 ± 208.16 The above-mentioned values were used to calculate the 8 4 2 1 124 8 following ratios (Kvet et al. 1971): (a) LAR: leaf area ratio; (b) SLA: specific leaf area; (c) AA: biomass allocation to Brown plants above-ground plant parts; (d) SA: biomass allocation to 770.17 ± 243.22 770.17 ± 243.22 stems; (e) LA: biomass allocation to ; (t) RA: biomass 2 557.00 ± 167.38 1114.01 ± 221.48 allocation to reproductive organs; (g) RoA: biomass alloca­ 4 266.95 ± 103.54 1087.79 ± 195.38 8 126.08 ± 79.86 909.61 ± 31.20 tion to roots. ANOVA, a one-way analysis of variance, was used to test 8 4 2 1 1 8 4 2 for a statistical significant difference at p = 0.05. Scheffe's test was used for paired comparisons between species and between treatments within each species (Steyn et al. 1987). index of the plant's leafiness (Causton & Venus 1981), and Statistically homogeneous groups are indicated under the the specific leaf area, an indicator of the leaf structure made tables and figures. Asterisks under the graphs indicate from the available dry material (Causton & Venus 1981) did statistically significant differences between plants of the not differ significantly (p = 0.05) between the different different diaspore types. densities for any of the diaspore types of the two examined Plants cultivated from disc and ray diaspores of D. sinu­ species (Table 3). Van Rooyen (1988) found the same for ata and black and brown diaspores of A. fastuosa are refer­ three other ephemeral species of Namaqualand. Plants culti­ red to as disc, ray, black and brown plants, respectively. vated from the different diaspore types of both D. sinuata and A. jastuosa did not differ significantly (p = 0.05) from Results and Discussion each other at any density regarding leaf area, leaf area ratio, Leaf area, leaf area ratio, specific leaf area and number specific leaf area and total number of inflorescences (Tables of inflorescences 1 - 3). The average leaf area per plant and the total number of inflorescences per plant of disc and ray plants of D. sinuata Biomass production and black and brown plants of A. jastuosa decreased signifi­ The total dry mass per plant of the disc and ray as well as cantly (p = 0.05) with an increase in density (Tables 1 & 2). the black and brown plants of D. sinuata and A. jastuosa, The leaf area and number of inflorescences per pot of disc respectively, decreased significantly (p = 0.05) with and ray plants of D. sinuata increased significantly (p = increasing density. Total dry mass per plant of plants culti­ 0.05) at low densities but stabilized between densities of vated from the two diaspore types of both D. sinuata and A. four and eight plants per pot. However, in both the black jastuosa differed significantly (p = 0.05) only at a density of and brown plants of A. jastuosa the increase in leaf area and two plants per pot (Figures la & Ib). The total yield per pot number of inflorescences per pot with an increase in density of the disc and ray plants of D. sinuata increased were not significant (Tables 1 & 2). The leaf area ratio, an significantly (p = 0.05) with increasing density. Ray plants S.AfrJ.Bot., 1992, 58(6) 463

Table 2 The influence of density on the total number of Table 3 The influence of density on the leaf area ratio inflorescences of Dimorphotheca sinuata and Arctotis and specific leaf area of Dimorphotheca sinuata and fastuosa. Statistical groups (p = 0.05) are indicated under Arctotis fastuosa. Statistically homogenous groups (p = each group. Data are means ± standard deviations 0.05) are indicated under each group. Data are means ± standard deviations Total inflorescences Density Density (plants per pot) Per plant Per pot (plants per pot) Leaf area ratio Specific leaf area

Dimorphotheca sinuata Dimorphotheca sinuata Disc plants Disc plants 1 46.75:!: 3.53 46.75:!: 3.53 1 45.69 :!: 11.63 236.42:!: 94.69 2 24.25 :!: 12.13 48.50 :!: 17.52 2 48.60:!: 8.06 225.78:!: 32.36 4 24.17:!: 12.00 95.17 :!: 24.79 4 52.85 :!: 13.25 260.38:!: 48.97 8 13.25:!: 4.55 105.80 :!: 11.75 8 49.68 :!: 14.83 260.30:!: 80.29 8 4 2 1 124 8 1 284 2 1 8 4

Ray plants Ray plants 1 36.67:!: 8.01 36.67 ± 8.01 54.78 :!: 25.06 269.89:!: 71.24 2 41.33 ± 18.00 82.67 :!: 13.84 2 41.58:!: 9.44 238.44:!: 34.13 4 23.30 ± 12.87 93 .20 :!: 19.30 4 47.66 :!: 12.26 234.23:!: 35.04 8 13.63 ± 7.75 109.00:!: 8.22 8 44.64 :!: 13.16 143.71:!: 40.42 841 2 124 8 2 8 4 1 428 1

Arctotis fastuosa A rctotis fastuosa Black plants Black plants 1 17.67:!: 4.85 17.67:!: 4.85 1 45.64:!: 14.19 170.11:!: 68.52 2 6.42:!: 3.33 12.83:!: 5.70 2 72.32 :!: 25.34 230.l6:!: 40.77 4 4.80:!: 2.68 19.20:!: 3.31 4 69.98 :!: 16.02 247.76:!: 42.98 8 2.90:!: 1.66 23.00:!: 4.15 8 71 .67 :!: 22.67 265.46 :!: 107.45 842 1 2 1 4 8 1 482 1 2 4 8

Brown plants Brown plants 1 13.33:!: 4.53 13.33:!: 4.53 1 57.68 :!: 17.83 224.34:!: 85.46 2 8.40:!: 3.47 16.80:!: 5.46 2 59.30:!: 7.09 224.73:!: 19.52 4 4.95:!: 1.99 19.80:!: 3.12 4 56.49:!: 9.52 219.71:!: 36.97 8 3.63:!: 1.63 29.33:!: 4.64 8 58.75 :!: 32.68 226.42 :!: 131.92 842 1 124 8 4 1 8 2 4 1 2 8

reached their plateau yield at a density of two plants per pot. gives a greater yield than higher and lower densities. When However, disc plants had not yet reached their plateau yield the yield per plant population has a parabolic curve, the at the highest density examined in this study (Figure la), yield is a function of reproductive growth, whereas the since the graph had not leveled out at a density of eight asymptotic relationship between yield and density is charac­ plants per pot. The plateau yield of the black and brown teristic of yields which are some product of vegetative plants of A. Jastuosa had already been attained at a density growth (Holliday 1960). of one plant per pot since there were no significant differ­ This constant yield over a wide range of densities is ences (p = 0.05) between the yields per pot at the different thought to represent the maximum fixation of energy that densities. the crop can achieve in the time between sowing and har­ When monocultures are planted over a wide range of den­ vesting (Bleasdale 1966). Shinozaki and Kira (1956) derived sities, intraspecific competition increases with increasing a linear relationship between the inverse of the average dry density resulting in reduced yield per plant (Jolliffe et al. mass per plant and density, called the 'reciprocal yield law', 1984). The increase in size variability in populations grown which describes the asymptotic relationship between yield at higher densities has been interpreted as strong support for and density: the hypothesis that competition between plants is one-sided, i.e. the larger plants are able to obtain a disproportionate W -1 =Ap +B, share of resources and suppress the growth of the smaller individuals (Miller & Weiner 1989). where W is the yield per plant, p is the density, and A and B According to Holliday (1960) the relationship between are constants. This relationship for the two diaspore types of density and yield can be either (i) asymptotic, where the D. sinuata and A. Jastuosa is shown in Figures 2a and 2b, total yield increases with density until a certain point, where respectively. it levels off; this is called the 'law of constant final yield' The dry mass of the different organs (stems, leaves, inflo­ (Harper 1977), or (ii) parabolic, where one particular density rescences and roots) per plant cultivated from the different 464 S.-Afr.Tydsla.Plantk., 1992,58(6)

~r------' ~r------~ a b .. I.

• • Density (plants per pot) Density• (plants per pot)• • Disc/plant DRay/plant ___ IlIac/pot -+- Ray/pot • Black/plant D Brawn/plant -><- Black/pot -+-llrown/pot Per Plant: Per Pot Per Plant: Per Pot: Disc: ~!t. ~ 1. Disc: 1. f. La Black: !l "- " 1 Black: li..L!l. Ray: !l."-U Ray: l~ Brown: !l "- " 1 Brown: ~ Figure 1 The influence of density on the above-ground dry mass per plant and per pot of (a) disc and ray plants of Dimorphotheca sinuala and (b) black and brown plants of Arctotis jastuosa. Bars represent standard deviations. Statistically homogeneous groups are indicated under the figures. ... a b x ...... j 'a 0.3 ~ '"m a~ ... eto ... •. 1

.~ ______-L ______L- ______-L ______L- ______~ • • • -a • • Density (plants per pot) Density (plants per pot) _ mack regression • Black _ Disc regression • IlIac _ Ray regression x Ray _ Brawn regression x Brown Disc: (y=O, O,053x + 0,070 r2:;:O,995) Black: (y=O, O,049x + 0,024 r2=O,986) Ray : (y=O, O.~62x + 0,053 r 2 =O,980) Brown: (y=O, O,059x + 0,007 z-2'=O,994)

Figure 2 The relationship between dry mass per plant and density of (a) disc and ray plants of Dimorphotheca sinuata and (b) black and brown plants of Arctotisjastuosa. diaspore types of both D. sinuata and A. Jastuosa decreased individual size (Thrall et ai. 1989) but biomass allocation is significantly (p = 0.05) with an increase in density (Figures also often affected (Van Rooyen et ai., in press). 3a - 3d). Ray plants produced significantly (p = 0.05) more Kira et ai. (1956) showed that stems, inflorescences and roots than disc plants at a density of two plants per pot. Disc plants of D. sinuata produced W=kW~ (1) significantly (p = 0.05) more inflorescences at a density of one plant per pot and more roots at a density of four plants which may be written in the linear form: per pot than ray plants. The only difference between brown and black plants of A. Jastuosa was found at a density of 10gW = logk + AlogWj, (2) two plants per pot where the stem dry mass of the brown plants was significantly (p = 0.05) higher than that of the where W is the mass of the entire plant, WI is the mass of a black plants. plant part, and k and A are constants. The relationship between yield and density seemed to be Bleasdale (1966, 1967) showed that in cases where the asymptotic for both the diaspore type plants of D. sinuata reciprocal yield law is valid, the value of A indicates the and A. Jastuosa. change in the ratio of the mass of the plant organ to the total dry mass of the plant at different densities. If A = 1, the Biomass allocation to plant parts organ is a constant proportion of the total dry mass, values Not only can plants adjust to density by plastic responses in greater than 1 indicate an increase in the organ with in- S.Afr.J.Bot.,1992,58(6) 465

a b

. . .- Del18!ty (planta per pot) Density• (planta per pot)• ~ Leave. 0 lDflorescencOIII II Roota EEj Leaves D Inflorescences III Roots Stems : Inflorescence: e. ~ z."r' Leaves: Roots : a !!. Z"l" Steas: Inflorescence: a ! U" Leaves: Roots: a!!. U"

c d

• • Del18!ty (planta per pot) Density• (planta per pot)• t:nLeavel o lD1lore&CenC8S II Roots ~ Leaves Q Inflorescences II Roots Steas : 6.U"l Inflorescence: a U 1 Steas: Leaves: a!!.Zl Inflorescence: .e. ~ 2. 1 Roots: a!!. ,,"1 Leaves : Roots: a!!. Z"l Figure 3 The influence of density on the dry mass of the plant organs per plant of the (a) disc and (b) ray plants of Dirrwrphotheca sinua/a and the (c) black and (d) brown plants of Arctotis fastuosa. Bars represent standard deviations. Statistically homogeneous groups are indicated under the figures . Asterisks denote statisticallysignificant differences between plants of the different diaspore types.

creasing density and values less than 1 a decrease in the of disc plants increased with increasing density (Figure 4a). organ with increasing density. The value of A for the stems of the black and brown The A and ,2 values of the different organs of both D. plants is less than 1, indicating that stem allocation decreas­ sinuata and A. Jastuosa are represented in Table 4. ed with increasing density (Figures 4c & 4d). In the case of The value of A is approximately I for stems, leaves and the leaves of plants of both the diaspore types the value of A inflorescences of both disc and ray plants of D. sinuata. approximately equals 1 and therefore leaf allocation is not These organs are, thus, not influenced by plant density (Fig­ influenced by density. The value of A for the inflorescences ures 4a & 4b). In the case of the roots the value of A is of the black plants approximately equals 1, and reproductive greater than 1 in the disc plants. Therefore, root allocation allocation is not influenced by density (Figure 4c), but in the

Table 4 The A and r values in parentheses of the organs of disc, ray, black and brown plants of Dimorphothecasinuata and Arctotis fastuosa

Stems Leaves Inflorescences Roots

D. sinuata Disc plants 0.98 (0.998) 0.97 (0.994) 0.98 (0.983) 1.14 (0.933) Ray plants 0.97 (0.999) 1.06 (0.990) 1.01 (0.994) 0.93 (0.992) A·fastuosa Blaek plants 0.93 (0.999) 1.03 (0.997) 0.98 (1.000) 1.37 (0.999) Brown plants 0.92 (0.999) 1.05 (0.994) 0.90 (0.993) 1.55 (0.969) 466 S.-Afr.Tydskr.Plantk., 1992, 58(6)

l~r--r~-----.-'------'--r------'--r------.--

• Density• (plants per pot)• Density (plants per pot)• • Roots ~ Stsms tI;:] Leave. D Inflorescenoe! • Roots ~ Stems ~ Leaves o Inflorescences Roots: 1 2 4 8 Leaves: 8 1 4 2- Roots: Leaves: z..:.u..!­ Stells: 48i! Inflorescences: U~ Ste.s: Inflorescences: L.a...!L.Z

100 ,-,------,---,-::------,---r------,...-,--,..---....---,

..

• • Density (plants per pot) Density• (plants per pot)• • Roots ~ steID8 ~ Leavell o Inflorescences • Roots fm Stems 0 Leaves [::J Infiorescences

Roots: Leaves: L!.....a ",­ Roots: 1 2 4 8 Leaves: ~. Steas: Inflorescences: a:.2....1 4 Stems: ~1 Inflorescences: ~ Figure 4 The influence of density on biomass allocation in the (a) disc and (b) ray plants of Dimorphotheca sinuata and (c) black and (d) brown plants of Arctotis jastuosa. Statistically homogeneous groups are indicated under the figures. Asterisks denote statistically significant differences between plants of the different diaspore types.

case of the brown plants A is less than 1 and reproductive Table 5 Species monoculture and relative monocul- allocation decreased with increasing density (Figure 4d). ture responses of Dimorphotheca sinuata and Arctotis The value of A was greater than 1 for roots, indicating that fastuosa at the various densities root allocation increased with increasing density (Figures 4c & 4d). spp. monoculture Relative monoculture Biomass allocation to plant parts of the two polymorphs Disc Ray Disc Ray at each density seemed fairly similar in both species (Fig­ ures 4a-4d). D. sinuata 2 2.48 -0.12 0.31 -0.02 Species monoculture and relative monoculture 4 4.19 3.37 0.53 0.50 response 8 5.95 5.00 0.75 0.74 The effect of density on monocultures can be evaluated Black Brown Black Brown following the method Jolliffe et al. (1984) propose for the interpretation of replacement series: Let Yp be the projected A·fastuosa 2 9.69 5.32 0.59 0.36 yield of a species per unit area, and Ym the monoculture 4 11.64 10.24 0.70 0.69 yield of a species per unit area, then species monoculture re­ 8 14.15 12.80 0.85 0.86 sponse will be equal to Yp - Ym , and the relative monocul­ ture response will be equal to (Yp - Ym)/Yp' The species monoculture response at any given planting density of a species is a measure of intramorphic competi­ increased with increasing density for plants of the different tion, whereas the relative monoculture response is the rela­ diaspore types of both D. sinuata and A. Jastuosa. tive effect of intramorphic competition on the yield of an Therefore, the influence of intramorphic competition is individual species in monoculture. greater at higher densities. Intramorphic competition was The species monoculture and relative monoculture re­ greater in the disc and black plants than in the ray and sponses for D. sinuata and A. Jastuosa are shown in Table 5. brown plants of D. sinuata and A. Jastuosa, respectively. The species monoculture and relative monoculture response However, the relative monocuIture response of both plants S.Afr.1.Bot., 1992,58(6) 467

of D. sinuata and those of A. Jastuosa did not differ at BAEUMER, K. & DE WIT e.T. 1968. Competitive interference densities of four and eight plants per pot. There is, thus, no of plant species in monocultures and mixed stands. Neth. f . difference in the relative effect of intramorphic competition agric. Sci. 16: 103 - 122. on the yield of the two diaspore types of D. sinuata and A. BEATLEY, J.C. 1974. Phenological events and their environ­ Jastuosa at these densities in monocuIture. mental triggers in the Mojave desert ecosystems. Ecology 55: 856 - 863. BENEKE, K. 1991. Fruit polymorphism in ephemeral species of Conclusions Namaqualand. MSc thesis, University of Pretoria, Pretoria. Although the germination behaviour of disc and ray dia­ BENEKE, K., VON TEICHMAN, I., VAN ROO YEN, MW. & spores of D. sinuata differed vastly (Beneke et al. 1992c) THERON, G.K. 1992a. Fruit polymorphism in ephemeral and a growth analysis of the disc and ray plants which had species of Namaqualand. I. Anatomical differences between been cultivated from intact achenes (Beneke et al. 1992d) polymorphic diaspores of two Dimorphotheca species. S. Afr. also indicated marked differences, the yield of disc and ray f. Bot. 58: 448 - 455. plants, cultivated from excised embryos, was only signifi­ BENEKE, K., VON TEICHMAN, I., VAN ROOYEN, MW. & cantly different at a density of two plants per pot. If the THERON, G.K. 1991b. Fruit polymorphism in ephemeral germination of the ray diaspores had not been improved by species of Namaqualand. II. Anatomical differences between polymorphic diaspores of Arctotis fastuosa and Ursinia cakile­ excising the embryos, the differences in yield between disc folia. S. Afr. f. Bot. 58: 456 - 460. and ray plants might have been larger. The species mono­ BENEKE, K., VAN ROOYEN, M.W., THERON, G.K. & VAN culture response and relative monoculture response (Jolliffe DE VENTER, H.A. 1992c. Fruit polymorphism in ephemeral et al. 1984) indicated that intramorphic competition was species of Namaqualand. III. Germination differences between stronger in monocuItures of disc plants than of ray plants, the polymorphic diaspores. 1. Arid Environ. (in press). thus disc plants are better in tram orphic competitors. Since BENEKE, K., VAN ROOYEN, M.W. & THERON, G.K. 1992d. disc diaspores of D. sinuata are the better germinators Fruit polymorphism in ephemeral species of Namaqualand. IV. (Beneke et al. 1992c) and produce plants that are stronger Growth analyses of plants cultivated from dimorphic diaspores. growers (Beneke et al. 1992d) and better intermorphic 1. Arid Environ. (in press). competitors (Beneke et al. I 992e), the disc diaspores are BENEKE, K., VAN ROOYEN, M.W. & THERON, G.K. 1992e. responsible for the relative abundance of the species and the Fruit polymorphism in ephemeral species of Namaqualand. VI. extention of its range. The ray diaspores with their delayed Intermorphic competition among plants cultivated from di­ germination produce plants of which diaspore production is morphic diaspores. S. Afr. f. Bot. 58: 468 - 476. BENEKE, K., VAN RooYEN, M.W. & THERON, G.K. 1992f. not influenced by moisture stress and sowing date (Beneke Fruit polymorphism in ephemeral species of Namaqualand. et al. I 992f). They are thus responsible for the survival of VII. Diaspore production of plants cultivated from dimorpic the species through unfavourable conditions and for the diaspores. f. Arid Environ. (in press). maintenance of a seed supply in the seed bank. BLEASDALE, J.K.A. 1966. Plant growth and crop yield. Ann. The yield of black and brown plants of A. Jastuosa in appl. Bioi. 57: 173 - 182. reaction to density was not significantly different, except at BLEASDALE, J.K.A. 1967. The relationship between the weight a density of two plants per pot. The lack of differences of a plant part and total weight as affected by plant density. f. between black and brown plants in this study reflects the hort. Sci. 42: 51 - 58. lack of significant differences in germination behaviour CAUSTON, D.R. & VENUS, J.e. 1981. The biometry of plant between the two morphs. The values of the species mono­ growth, pp. 1 - 307. Edward Arnold, London. culture response and relative monoculture response showed FIRBANK, L.G. & WATKINSON, A.R. 1985. On the analysis of that intramorphic competition was stronger in monocultures competition within two-species mixtures of plants. f. appl. of black plants than of brown plants. Ecol. 22: 503 - 517. Polymorphism is a fairly common phenomenon among HARPER, J.L. 1967. A Darwinian approach to plant ecology. f . the ephemeral species of Namaqualand and many of the Ecol. 55: 247 - 270. species producing mass displays are polymorphic. The ad­ HARPER, J.L. 1977. 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