212 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



Plants that grow under favourable conditions of water-supply 

 are termed Hygrophytes. 



(/) Means to check too much Transpiration.— r^ Too much 

 transpiration, /. e., too great a loss of water is, on the other 

 hand, dangerous for the life of a plant. Plants, which have to 

 live on very dry soil, or are exposed to the scorching heat of the 

 sun or to the parching influence of dry winds, or which for 

 many months must do without a drop of rain, must needs have 

 some means of reducing the action of transpiration. 



Above all, we notice that the leaves of such plants have a 

 very limited surface (cf. Leucas, page 104), and are sometimes 

 reduced to mere spines or scales (Cactus, page 58; Casuarina, 

 page 181). Some plants drop their leaves entirely during the 

 the dry season (Teak, page 112; Silk Cotton tree, fig. 195) to 

 prevent loss of moisture by evaporation. 



Another kind of protection from the scorching heat of the 

 sun is the vertical position of leaves, as is seen in many trees of 



Australia, of which the Eucalyptus 

 tree is one. Instead of holding their 

 leaves flatly or horizontally, as trees 

 generally do, so as to catch every 

 ray of sunlight, they avoid the heat 

 as much as possible by holding them 

 edgeways to the light. Similarly 

 some other plants, like Oxalis [Kan. 

 Pullampunise; 2Ial. Puliyfiral: San. 

 Cukrika) fold their leaves at midday 

 when the sun shines hottest. 



Various plants that live on dry 

 and rocky soil, store up in iheir /lesJiy 

 stems and leaves every drop of water 

 they can get when rain falls, and live 

 si)aringly on it during the long per- 

 iods of drought which may last for 

 three-quarters of the year. Such 

 plants are the Pryophyllum {Kan. ivadu-basale), the Agave 

 (Kan. Anekattfili). and many Spuri^^es and Cactuses. A number 



c. 



Kij;-. r.M). — Solar rays strikiuii;- 

 the lino " '/ at rij;lit aii^j^los and 



heatintf it much more tlian the 

 lines a h or a c which lie more in 



the direction of the sun's rays. 



