- EXPERIMENT 26 STUDY OF XEROPHYTES, MESOPMYTES AND I I HYDROPHYTES I Structure 26.1 I~ilroduction Objecrives 26.2 Categories 01 PlanLs based on Habitat 26.3 Hydrophytcs Morophological Features Analomical Fcatures 26.4. Mesophytes 26.5 Xerophytes Morphological Features Anatomical Fcaturcs 26.6 SAQs 26.1 INTRODUCTION Tllis distribulion of plants occur Jsom exlrcmcs oJ north and soul11 and east and wesl around Ihe world. Thus certain groups or plants arc adaptcd to unusual or extreme climalic conditions, Tllcqc plants exhibit modificalions in heir anatomy and morpllology hat are relaled LO spccialised Junctions LO a greatcr degree. Tlle ecological groups 01 plants havc much broadcr group. But Lhcy havc to be classificd on some basis. Watcr is onc hem. Warming (1909) classificd plant communilies on tlle basis of Lheir dependence upon and rclalion to watcr. Water bcing most important occupics forenlosl position in distribution of vegetation and its structure. Warnling primarily recogniscd ll~recmajor groups of plants: i) Hydrophy~es,ii) Mcsophytes and iii) Xcrophytcs. IIydrophytcs arc thosc plants which l?ve partly or 1uUy submerged in water at lcast for somc wccks. Mcsophytes grow on land wilh a moderate supply of soil moisture. Xcrophytcs are zhase plants lhat live in dry or arid land. Aftcr doing this cxcrcisc you will be able to: a describe and diffcrcnciatc bctwccn hydrophytcs, nicsophytcs and xcrophy~cs, a list varidus adaptations in hydrophyles, mcsophytes and xerophytes wilh cxamples and illustrrllions. 26.2 CATEGORIES OF PLANTS BASED ON HABITAT In Lhis cxpcrirncnt you will bc providcd spccimcns and slidcs. From the givcn description you havc to classi1y Lhc plants into differcnl catagorics (dcscribcd bclow). Laboratory Course-I According lo thc way In whicl~lhc hgdroph~[thd~\~lo[> in MAICI. [hey arc: subdivided into lhc following five ca[agolius: i) ' Free-Floating IIydrophytes Thcsc typcs of pla~ilsrcmain in conlacl will1 walcr and air, but no1 soil. Thcy float frccly on thc watcr surfacc. Lcavcs in somc arc vcry minulc wliilc in othcrs quitc largc. Somc of thc frcc flpating hydrophylcs arc Trupa hi.spi:no,su, Azolla, Eiclzhornia crassipes, Salvinia, Wolffia, Pistia, Lcmna (Fig. 26.1). Fronds - - - - - . - - - - - . - - - - - - - -- - - - - -- - - - - - - - -. Water :.T ==r-z--z--:x-:: WOl/~~~l Frond (Plant body) Root .- ----.- -- Pockcl - - Eichhornia Lrmna Frond Floating Leaves~~ ~ Leaves A"rC?/7 Water ~ Stem .:: Azolla Pisria Fig. 26.1 : Free floating hydrophytes ii) Rooted Hydrophytes wit11 Floating Leaves The roots of these type of hydrophytes are fixed in mud, but leaves have long petioles which keep thcm floating on the water surhce. Except leaves, the rest of the plant body remains in water. Some examplcs are Nelumbo nucvib-a, Nymphaea stellate (water lily), Trapa, Marsilea (Fig. 26.2). Study of Xeropnyres, Mesophytes and Hydrophytes Flower Fig. 26.2 : Rooted hydrophytes with floating leaves . iii) Submerged Floating Hydrophytes mese types of the plants are only in contact with water, being completely submerged and not rooted in the mud. Their stems arelong and leaves generally small. Some examples are Ceratophyllum, ULn'culmiu. (Fig. 26.3) -------- ---_U __ ___ Ceratophyllum Fig. 26.3 : Submerged floating hydrophyte iv) Roolcd aull~~~cr.gcclhydrophytes This type of hydrophytes rcrnain completely submerged in water and'rooted in soil. In somc plants the slcm is long, bearing small leaves at the nodes. In some planls stcm is tubcrous (corn-like) with long leaves, which are narrow, ribbon-shaped. Common example is Vallisneria, Chara, Hydrilla, Potamogeton (Fig. 26.4) Flower Water Submerged Leave> .- Hydrilla Flg. 26.4 : Rooted submerged hydrophytes v) Rooted emergent hydrophytes These types of plants grow in shallow water. These are such hydrophilous forms which require excess of water, but their shoots (assimilatory organs) are partly or completely exposed to air. The root system is completely under water, fixed in soil. In some plants, shoots are partly in water and partly emerging i.e. exposed to air. Whereas in some the shoots are completely exposed to air. Some common examples are Sagittaria, Ranunculus, Cyperus (Fig. 26.5). Now let us see the ecological adaptations in hydrophytes. Though most of the features in hydrophytes are similar but they may differ from each other in some : aspect. We will discuss all those features which enable them to become hydrophytes. 26.3.1 Morphological Features 1. Roots There is plenty of water in the surroundings of hydrophytes; thus the root becomes of secondary importance hence less developed and insignificant in . most of hydrophytes. Study of Xerophytes, Mesophytes and Hydrophytes 1 Emergent , Fig. 26.5 : Rooted emergent hydrophytes i) Roots may be completely absent or poorly developed as in Wolflia, Salvinia. However, in emergent forms, which grow in mud, mots arc well developed with distinct root caps. ii) Root hairs are absent or poorly developed. iii) Root caps are usually absent, in some cases are replaced by root pockcts as in Eichhornia. iv) Roots when present are generally fibrous, adventitious, reduccd in length, and unbranched or poorly branched. 2. Stems i) In emerged forms the stem is long, slender, spongy and flexible. In free-floating forms it may be slender, floating horizontally on water surfacc or thick. short, stoloniferous and spongy. In forms which are rooted with floating leaves it is rhizome. ii) Vegetative propagation is by runners, stolons, stem and root tubers, dormant apices, offsets etc. are the common merhod of reproduction. Most or them are perennials. 3. Leaves 'i) In submcrged forms, leaves are thin, and are eithcr long and ribbon-shaped as in Vallisneria or long and linear or finely dissected as in Potamogeton. Floating leaves are largc, flat and entire with their upper surfaces coated with wax as in Nymphaea; their petioles lung, flexible, and often covered with mucilage. In some cases petioles become swollen and spongy as in Eichhornia. ii) Emergent forms show heterophylly with submerged, floating and aerial - leaves as in Ranunculus. iii) Submerged leaves are generally translucent. 4. Flowers and seeds In submerged forms they are less common. Where flowers develop, seeds are rarely formed. 26.3.2 Anatomical features Now, wc will study aboul: the anatomical features of roots of the hydrophytes. 1. Roots i) Cuticle is either completely absent or if prcsent it is thin and poorly developed. ii) ~piderniisis usually single layered and made up of thin wallcd parenchymatous cells. iii) Thc cortex is well developed, thin walled and parenchymatous, major portion of which is occupicd by well developed prominent air cavities-tlie 'arenchyma' which offers resistance to binding slress, increases buoyancy and allows a rapid gaseous exchange. iv) In the given figure of Potamogeton you can see that va~cular~tissuesare poorly dcvelopcd and leasl dirrcrcntiated in subrnergcd form. The xylem vessels are lcss common and tfachcids are present. In floating types vascular tissue are less dcvclopcd while in emergent fonns they are much distinct and wcll devclopcd (Fig. 26.6). v) Thc mechanical tissues are absent except in son?e %ergcnt forms where pith is made up of sclcrcnchymatous cells. OLUVJ Ul AG1UpllJ.Y Mesophytes and Wydrophytes Air Chambers , Fig. 26.6 : TS. root of Potarnogetorr pectinnlus (submerged hydrophyte). Note the absence of root hairs and cuticle; undifferentiated broad cortex with alr chambers; vascular tissues poorly developed, represented rnalnly by phloem; lack of mechanicnl tlssues. I 2. Stem I t In Lhe given iigurc of T.S. of stcm of Hydrilkz you can sce that Fig. 26.7) Air Chambers Fig. 26.7 : TS. stem of H-vdrilla (submerged hydrophyte). Note, the absence of cuticle; thin- walled epidermis; ~ndlfferentiatedcortex wlth alr chambers; abundance of thin-wnlled elements; absence of tire( hanicsl tissues; reduced vascular elements, composed chiefly oP phloem, xylenl being represented only by a cavlty In the centre. Laboratory Course-I -" i) Cuticle is developed or thin or endrcly absent. ii) Epidermis is single layered and made up of thin-walled parenchymatous cells while in emergent form cuticle as well as epidermis is generally wcll develop such as Typha. iii) In'floating and emergent form you can see hypodermis may be present as thin-walled parenchyma or collenchyma. It is completely absent in submerged form. iv) Cortex is well developed. One of the important feature of the cortex is that it is thin walled and parenchymatous extensively transversed by air cavities. The cortical cells generally possess'chloroplasts and are photosynthetic. v) Endodermis is distinct, especially in rhizomes and similar organs. vi) Generally, vascular bundle have no bundle sheaths. Vascular bundlcs arc 4 thin walled. But in emergent forms vascular elements are comparatively well differentiated and developed. vii) Mechanical tissues are generally not present. 3. Leaves You can see form the slides provided to you of different T.S. (of various plant leaves.) You can see that internal structure of leaves show variation but some of the anatomical feature is common to the most of Lhc Icavcs. From the given transverse section you can see that; i) Cuticle is usually absent in submerged forms such as Potamogeton but in floating forms it is poorly developed confined to upper side and is very thin. In emergent form also cuticle is thin (Fig. 26.8). Mesophyll Cells ,Upper Epidermis h Lower Epidermis Fig. 26.8 : T.S. leaf (only laternl wing portion shown) of Potamogeton pusillus (submerged). Note, the absence of cutkle and stomata undifferentiated single-layered mesophyli between two epidermnl layers. ii) Epidermis is single-layered, made up of thin-walled cells with abundance of chloroplasts. ! iii) Stomata are completely absent in submerged leaves, as in Potamogeton. In floating form stomata are confined only to the upper surface of leaf, whereas in emergent forms they are generally found on both of the surfaces of leaves. iv) Mesophyll is undifferentiated in submerged leaves, and generally it is single layered.