82 INTRODUCTION TO PLANT GEOGRAPHY 



relation to the needs of plants to combat adverse conditions. 

 Through such structural changes they may be enabled to maintain 

 their geographical areas and even to extend them. Those modifica- 

 tions of reproductive bodies that are helpful in dispersal will be 

 dealt with in the next chapter, the present section being concerned 

 primarily with the ' vegetative ' parts — comprising, in higher plants , 

 the stems, roots, and leaves. 



The water relationships of plants often involve strikingly ' adap- 

 tive ' features — particularly ones that are helpful in tiding over 

 periods of water deficiency, for example by increasing absorption 

 or decreasing loss, or by storage against times of need. Instances 

 are seen in the deep roots of many plants of deserts or semi-deserts, 

 allowing the tapping of underground reserves, and in the matted 

 turf of the Grasses of semi-arid regions, which aids retention of such 

 water as becomes available from atmospheric sources. Actually, as 

 pointed out by Professor Kenneth V. Thimann {in litt.), ' roots 

 elongate when aerated ; hence in dry soils (which are therefore full 

 of air) they grow longer. ... I should call [this] a simple response 

 to external conditions. Low nitrogen also favors elongation of 

 roots, with obvious ecological advantages in nitrogen-poor soil.' 



The aerial parts of a wide range of plants are modified to reduce 

 water-loss, often to the slightest proportions in times of shortage. 

 This may be done, for example, by protection of the stomata in grooves 

 or among a mass of hairs, by general reduction of the ' evaporating 

 surface ', and by covering with wax or hairs, etc., even those areas 

 that remain. Often the leaves are reduced to spines or scales, their 

 normal functions being taken over by green stems. In addition 

 many plants, such as the more massive succulents of the Cactus, 

 Spurge, and some other families, store water extensively in special 

 stem or other structures which are modified into reservoirs. There 

 may also be one or more layers of large water-storing cells in leaves 

 and other green parts. The development of some of these features, 

 such as the tall stems of many trees in dense forests, may depend 

 upon the conditions under which an individual grows, whereas in 

 other cases the features may develop regularly, irrespective of local 

 conditions, as part of the normal form of the plant. But in either 

 instance the ' ability ' has to be present, else the plant could not 

 develop the desirable adaptation and benefit accordingly. Fig. 20 

 shows some examples of features that help land plants to conserve 

 or obtain water ; conversely, many water plants have special tissues 

 or growths that enable them to float or otherwise improve their 



