IMPORTANCE OF WATER. 263 



quite decisive as to the possibility of vegetation. The deserts of Asia and Africa owe 

 their paucity of plants essentially to the drought prevailing there; since around 

 every spring by chance occurring even in the desert, a luxuriant oasis of vegetation 

 becomes developed, simply because all other nutritive materials are present in the 

 sand of the desert and the water of the spring. Water also affects the whole 

 organisation of the plant in a definite manner. That submerged and floating aquatic 

 plants in general present a different aspect from land-plants, and are of a more delicate 

 and simpler structure, strikes every observer at once. To lay stress on one point 

 only; it is obvious that land-plants, the large green leaf-surfaces of which are 

 extended in dry air for the purpose of producing vegetable substance under the 

 influence of sunlight by assimilation, are necessitated not only to absorb the salts of 

 the soil which co-operate in this process by means of roots, but also to transport 

 them into the assimilating leaves. This takes place, however, by means of a current 

 of water ascending from the roots through the stem and branches into the leaves ; and 

 this is maintained by means of the continuous evaporation from the leaves. This 

 ascending current of water now requires a special organ in which to move, and that is 

 the woody body; it also requires a richly-developed system of roots to collect the small 

 quantities of moisture of the soil, which contains but little water, and so forth. These 

 arrangements, as is at once obvious, are superfluous in a submerged water-plant ; and, 

 therefore, proper wood is wanting to it, and its roots are insignificant in comparison 

 with those of a land-plant. It scarcely needs special mention that the most various 

 intermediate forms exist in the connection referred to; and these are still more 

 various because nature generally can attain her object by very different means. 

 Under conditions, for example, where the transpiration of leaves is too copious, forms 

 appear without leaves, as the majority of the species of Cactus, and similarly formed 

 Euphorbiaceae and Stapelias ; or forms with thick succulent leaves like the Crassu- 

 lacese, in which transpiration is likewise only very feeble, or, finally, woody shrubs 

 with few and small leaves. But just the most interesting and instructive arrange- 

 ments which would here present themselves must be passed over for the time being, 

 since they require too lengthy a description. 



Just as plants react towards cosmical and inorganic influences generally, so 

 also they exist in a relation of dependence towards various other plants and animals. 

 In this, again, they react in such a manner that the external shape of their bodies 

 and their internal organisation become modified in the most various ways. This is 

 evident in a very conspicuous manner in the life of parasitic plants. When they 

 absorb the whole of their food from other plants, or occasionally even from animals, 

 they themselves do not require leaves containing chlorophyll : they are, therefore, 

 devoid of leaves, and accordingly, agreeing with what has already been said, the 

 development of wood also is suppressed. ' The majority of parasites are massive 

 bodies of tissue, abounding in parenchyma, and with little development of surface, 

 the strange aspect of which usually strikes even the non-botanical observer, not to 

 speak of the further abnormalities, particularly of the sexual organs, which arise from 

 parasitism. Qn the other hand, however, even plants which contain chlorophyll and 

 are self-supporting may not only be enfeebled in vigour by parasitic plants, but also 

 altered in form. Of this, the Euphorbia infested with Fungi, as well as the so-called 

 Witches'-brooms (branches of a Fir altered by Fungi) offer well-known examples. 



