320 



ZONES AND REGIONS 



[Pt. Ill, Sect. I 



Ericaceae, Utriculariae, the young fig-tree (Fig. 160), also the spindle- 

 shaped swollen petioles of Philodeiidron cannaefoliuin, and the older, 

 yellowish, greatly thickened leaves of species of Peperomia and of Gesne- 

 raceae, supply the plants to which they belong with water, so that, as has 

 been proved experimentally, they thrive without receiving any water from 

 outside so long as this supply is available, but rapidly wither whenever it is 

 exhausted. 



Whilst the roots of many proto-epiphytes do not differ essentiallj' from 

 those of terrestrial plants, others are provided with a mechanism by which 

 every drop of falling water is at once absorbed. This takes place by means 

 of the vclamcn. a tissue that covers the roots of nearly all epiphj'tic orchids 



(Fig. 161) and of certain epiphytic Araceae. 

 The cells of this tissue, which is usually com- 

 posed of several layers, resemble tracheids, are 

 provided with spiral bands, and in dry weather 

 contain air. They are bounded within by an 

 endodermls (exodermis), some of whose cells 

 are differentiated as passage-cells. If water 

 reaches the root it is sucked up b)' the \elamen 

 as if by blotting-paper, and fills the cell-cavities. 

 Thence it passes more slowly through the pas- 

 sage-cells into the interior of the root. 



The roots of the epiphytes of this group are 

 for the most part exposed to light and con- 

 sequently often contain chlorophyll. This cir- 

 cumstance has caused one of the most peculiar 

 adaptive features among epiphytes, namely the 

 differentiation of the root-s\'Stem as the sole 

 organ of assimilation, with at the same time an 

 atrophy of the leaves. Such assimilating roots 

 either creep along the bark or hang freely down 

 through the air; they are, in man)- cases, dorsiventral (Fig. 114), and, to 

 permit the interchange of gases, they are always provided with permeable 

 places which are absent from the weakly assimilating roots of other orchids. 

 To the henii-epiphytes belong chiefly very large, partly arborescent forms, 

 such as the epiphytic fig-trees, species of Clusia and Carludovica, and large 

 Araceae belonging to the genera Philodendron and Anthurium. At first 

 the hemi-epiphyte behaves like one of the proto-epiphytes and develops 

 similar water-reservoirs. Its roots are of one kind, and all serve as 

 anchoring and absorbing organs. Later on, as in the root-climbing 

 Araceae^, there sets in a sharp differentiation of short anchoring-roots 

 and long absorbing-roots, and the absorbing-roots descend to the ground, 



' See p. 313. 



Fig. 160. FicMS sp. Epiphyle. 

 A'oung plnnt with tuber. Natural 

 size. After Went. 



