How Plants Resist Hostile Forces Around Them 269 



explains no doubt the reason for the prevalence of the large cell- 

 cavity in the construction of plant cells. But many of the plants 

 of dry places develop great numbers of specially-large cells ob- 

 viously adapted to water storage in particular, and the presence 

 of such cells makes the parts that contain them swollen, rounded, 

 soft-textured and translucent. This is the origin of the type of 

 plant-structure commonly described as succulent, and distinctive 

 of many Cactuses, Euphorbias, Mesembryanthemums, House- 

 leeks, and others, all of which grow either in deserts, or in other 

 places, such as the clefts of rocky hills, where water is scanty for 

 long times together. This storage of water is naturally com- 

 bined in a great many cases with the storage of food, in which 

 case the parts display a firmer texture and whiter aspect in 

 section, as illustrated for example by the Century Plants. And 

 the examples above given show that the storage organs can be 

 leaves, as well as stems, while roots are frequently used for the 

 same purpose. 



The minimization of transpiration is the third and most im- 

 portant of the protective adaptations against dryness. We have 

 noted already the method by which ordinary plants are protected 

 against drought, viz., the possession of a waterproof epidermis 

 whose only openings, the stomata, are protectively guarded. 

 Now there is apparently no limit to the thickness and perfection 

 of waterproofing that can be given by plants to their epidermis; 

 and if it were possible for them to exist without the stomata, 

 then plants in dry places could wrap themselves up in a way to 

 conserve their indispensable water without limit. But as the 

 reader well knows, green plants in order to live must have food, 

 which is made by photosynthesis, which requires a supply of 

 carbon dioxide, which must be drawn from the atmosphere out- 

 side. Thus is necessitated the existence of the stomata, which 

 must be open for a time and extent directly proportional to the 

 food to be made; and this means that water will escape, or tran- 

 spire, incidentally but inevitably, through those openings to an 



