NATURE OF PLANTS 43 



the Spanish bayonet, aloes, grease wood and sage with that of 

 our leafy plants. The cactus represents one of the extreme forms 

 of reduction. The leaves have been dispensed with entirely and 

 are not represented save possibly by the spines. Consequently 

 the work of the leaves devolves upon the stems. But even 

 with the reduction of surface and the development of thick- 

 ened, epidermal cells, cuticle, hairs, wax, etc., which cause the 

 tough, leathery character of the organs, these plants would not 

 be able to withstand the drying heat of the desert were it not 

 for the fact that the cells are rich in mucilaginous substances 

 and various salts which effectually retain the water. While 

 desert plants receive very meager amounts of water, this is so 

 well protected that the plants are usually rich in water. Drink- 

 ing water is frequently obtained in desert regions from cacti 

 by pounding up the pulpy interior and squeezing out the water. 

 Animals are well aware of this rich storehouse and will eagerly 

 devour the cacti after the spines are burned ofif. The develop- 

 ment of these storage cells accounts for the fleshy character of 

 many alkaline, saline, and desert plants. Curiously enough there 

 are many examples of xerophytic plants living in bogs and 

 marshes, as, for example, the rushes and sedges, the horse tail 

 ferns (Equisetum), the lamb kill (Kalmia) and leather leaf 

 (ChamcBdaphne) , etc. The cause of the association of these plants 

 with aquatic forms is not known. In exposed moors and heaths 

 these reduced forms would have decided advantage because of 

 their protection against drying winds. Our sedges and rushes 

 are exposed to very intense heat and light which may possibly 

 cause so heavy a transpiration that these plants are not able 

 to meet the loss by root absorption. That these plants may 

 have some difficulty in absorbing sufficient water, owing to a 

 lack of proper gas interchange in the roots, is indicated by the 

 curving up in the air of the roots of the bald cypress (Taxo- 

 dium) to form the so called knees. These outgrowths are com- 

 posed of loose spongy tissues and probably supply the roots with 

 air. At least they are not formed when the trees grow in suffi- 

 ciently dry and aerated soil. For the same reason the roots of 



