SECT. II 



PHYSIOLOGY 



195 



off by an equally heavy climbing plant (Aristolochia). Instead of the stem the 

 leaves themselves may become succulent, as in the House-leek and other species of 

 Sempcrvmun, also in many species of Sedum, 

 Aloe, and Agave. Both stem and leaves are 

 equally succulent in many species of Mesem- 

 bryanipemum. In other plants, the paren- 

 chyma of their stem tubers (epiphytic Orchids) 

 or of their thickened roots (Oxalideae) serve 

 as water-reservoirs. Epiphytic Bromeliaceac 

 catch the rain-water in reservoirs formed by 

 their closely-joined leaves, and then eagerly 

 take it up through the scaly hairs which cover 

 the leaf surfaces, as in species of Tillandsia. 

 Again, many epiphytic Orchids and Aroids 

 collect the rain-water in a swollen sheath de- 

 veloped from the epidermis of the aerial root 

 (velameii radicum, p. 100). In the case of 

 other epiphytic Orchids, Aroids, and Ferns ^ 

 {Asplenium Nidus, for instance), the humus 



and other material caught in receptacles formed FlG - lsl -— Euphorbia globosa. The re- 



by the leaves or aerial roots act like a sponge d , uc , ed lea ™ s "^ be seen on tlle u PP er 

 . . , . n . . , ., ° globose shoots, 



in taking up and retaining water, while the 



absorptive roots penetrate into these moist, compost -like masses and absorb 



both water and nutrient substances. Many species of Frullania (a Liverwort 



common on Beech trees) possess, on the other hand, special water-sacs on the 



under side of their thallus (Fig. 319). A particularly remarkable contrivance for 



maintaining a constant supply of water is exhibited by the epiphytic Dischidia 



Bafflesiana, a number of whose leaves form a deep but small-mouthed urn, into 



which the roots grow. It would seem at first sight unnecessary that plants like 



the Mangrove tree, which stand with their roots entirely in water, should require 



protection against too rapid transpiration ; but, as this tree grows in salt or 



brackish water, it is necessary to reduce the amount of water absorbed, in order to 



prevent a too great accumulation of salt in the tissues. 



The Absorption of Carbon (Assimilation) 



In any attempt to distinguish the relative importance of substances 

 utilised in plant nutrition, carbon undoubtedly ranks first. Every 

 organic substance contains carbon, and there is no other element which 

 could supply or take part in the formation of so many or such a variety 

 of substances, both in living organisms and in the chemical laboratory. 

 Organic chemistry, in short, is merely the chemistry of carbon 

 compounds. 



It requires no chemical analysis to realise that plants actually 

 contain carbon, although in an imperceptible form. Every burning 

 splinter of a match shows, by its carbonisation, the presence of this 

 element. An examination of a piece of charcoal in which the finest 

 structure of the wood is still distinguishable, shows how abundant is 

 the carbon and how uniformly distributed. Estimated by weight, the 



