96 _ BOTANY OF THE LIVING PLANT 



by evaporation must be organised. This is clone in various ways. 

 The most direct is by reduction in the proportion of surface to bulk 

 by assuming a rounded form of the stem with reduced leaves as in 

 Cactus (Fig. 71) ; or of the leaves themselves, as in Stonecrop. In such 

 plants the cuticle is found to be very well developed, while a waxy 

 coating over the surface m^kes it a still better protection. The 

 stomata are usually sunk in pits below the surface, while additional 

 protection may be given by hairy or scaly coverings. (Compare 

 Figs. 49, 50.) On the other hand, in certain plants exposed to sudden 

 strong insolation, undue loss is prevented by permanent curling of 

 the leaf with the stomatal surface within the hollow space, as in the 

 Heath Family ; or the curling of the leaf may be controlled by an 

 automatic mechanism, giving protection in dry air, but opening out 

 flat when the air is moist, as in many Grasses (see below, p. 156, 

 Fig. 115). Plants thus modified in relation to drought commonly 

 have a hard stiff texture of the shoot, often with spiny developments. 

 To such plants the term Xerophyte is applied. 



Such adaptive changes as these profoundly affect the general habit 

 of individual plants ; and the whole vegetation of a district may be 

 characterised by them, giving a prevalence of hard and spiny bush, 

 or of succulent herbage. There is no circumstance of life which im- 

 presses itself so deeply on the conformation of the plant as the water- 

 relation. If, as we have reason to believe, vegetation was originally 

 aquatic, in which condition there is no pressing water-problem, the 

 spread of plants to the land opened new risks and physiological 

 difficulties, while it presented, on the other hand, very positive 

 advantages. The various adaptations which plants show in meeting 

 the danger of undue loss by transpiration present to the mind the 

 results of a struggle against circumstances, which could only be met 

 by such means. But still it should be kept clearly in view that 

 transpiration in moderate degree is essential to the well-being of land- 

 living plants ; for it is the means of leading the food-supplies from 

 the soil in solution to the required spot. 



It is in solution that all of these supplies come. There is in the 

 normal plant no ingestion of food in mass, followed by gradual diges- 

 tion. All food is acquired by the plant molecule by molecule. 

 This is in contrast to animal nutrition. The Amoeba envelops 

 solid particles of food in its naked protoplast, absorbs from them 

 what it can, and ejects what it cannot digest. But in the plant 

 the vegetative protoplasts are habitually encysted b^ a cell-wall, 

 so that ingestion of solid particles is impossible. It follows directly 



