168 



BOTANY 



PART I 



contrary, require to facilitate the giving off of water from the aerial 

 shoots. Only in this way can a sufficiently active movement of 



. ' water from the roots in the soil 



to the organs above ground be 

 ensured to supply the requisite 

 quantity of nutrient salts. Many 

 hygrophytes, especially those that 

 inhabit the moistest situations, 

 resemble water plants in form and 

 structure. 



Hygrophytes show a variety of ar- 

 rangements to favour transpiration such 

 as expanded thin leaf-blades, thin cuticle, 

 and the situation of the stomata on 

 exposed projections raised above the 

 general surface. There are also peculi- 

 arities in their leaves which, as STAHL 

 showed, tend to get rid of the water after 

 heavy rainfall as quickly as possible. 

 Thus a drawn-out tip to the leaf-blade 

 (DRIP-TIP) or waxy coatings rendering 

 the surface of the leaf unwettable 

 facilitates the shedding of water from 

 the leaf; while a velvety surface, due 

 to the presence of papillae, spreads drops 



of water by capillary action into an extremely thin film which readily evaporates. 

 According to STAHL also the presence of pigments which absorb the rays of 

 light and heat falling on variegated leaves raise the temperature of the leaf and 

 maintain transpiration even in a saturated atmosphere. In guttation or the giving 

 off of drops of liquid water from water-excreting organs or HYDATHODES, some of 

 these plants have the means of giving off sufficient water when transpiration is com- 

 pletely stopped. These organs are glandular surfaces or hairs which secrete water, 

 or are special clefts in the epidermis through which water derived from the vascular 

 bundles is forced (cf. Fig. 131). 



(b) Adaptations to physiologically dry Habitats or to dry 

 Climates. Xerophytes ( 84>87 ). Plants, the shoots of which are 

 exposed to dry air while they have difficulty in obtaining an adequate 

 or sufficiently rapid supply of water to make good the loss in trans- 

 piration, require arrangements to diminish the latter process. The 

 ordinary limitation of transpiration by closure of the stomata is not 

 sufficient in the case of plants of exceptionally dry habitats or 

 climates. Only a few cormophytes can withstand drying up, as do 

 many Lichens and Bryophyta (cf. p. 222), and most of them die when 

 wilting is carried far. 



Plants with such arrangements to diminish the loss of water are 

 termed XEROPHILOUS or XEROPHYTES. They are recognisable by 

 their general habit. The morphological peculiarities which are 



FIG. 189. Stilt-roots in Rhizoplwra mucronata 

 in the Malay Archipelago. (After KARSTEN.) 



