66o 



ECOLOGY 



Possibly some epiphytes have chemical as well as mechanical relations with trees, 

 certain bark substances, perhaps, furnishing a necessary food element or neutralizing 

 root excreta; perhaps the bark of some trees may contain substances injurious to 

 the roots of certain epiphytes. 



The advantages and disadvantages of epiphytism. — It is difficult 

 to see any great advantage in the epiphytic habit other than that 

 epiphytes are relatively exempt from the extreme overcrowding which 

 characterizes soil plants. It is usual to class epiphytes with lianas as 

 a group developed in the " struggle for light," but there is no evidence 

 therefor; furthermore, epiphytes occur chiefly on trunks and branches 

 which are lighted scarcely better than the ground. The disadvantages 

 of the epiphytic habit, namely, restricted absorption and exposure to 

 high transpiration, are very obvious. The underlying causes of epiphy- 

 tism are unknown, though facultative epiphytes, 

 J * J such as Nephrolepis, suggest possible beginnings. 



f^''^tjt "^^^ bromelia series (p. 6i6) is continuous from the 



non-epiphytic pineapple through the leafy species 

 of Tillandsia (as T. utriculatd) to the leafless T. 

 usneoides and may represent a line of progress 

 toward obligate xerophytic epiphytism. 

 (11 Carbohydrate synthesis and aeration in stems. — 



Stem chlorophyll. — In addition to being organs of 

 foliage display, stems are important food making 

 organs. Young woody stems, as well as herba- 

 ceous stems, commonly are green, though the total 

 expanse of chlorophyll tissue in stems is much less 

 than in leaves. The chlorophyll occurs in the 

 cortex, gradually decreasing inwards, as in thick 

 leaves; sometimes (as in most leafless stems) the 

 outermost layers are differentiated into palisade 

 cells. As in leaves, there are internal air chambers 

 and stomata, which act as passageways for gases. 

 In many water plants there are capacious air 

 chambers, which are separated from one another 

 by diaphragms (fig. 792) or by solid nodes that 

 strengthen the stem. During the first year a thick 

 bark develops in woody stems, largely through the 

 formation of new cells and the subsequent modification of their walls. 

 The stems in most instances no longer appear green ; nevertheless be- 



^ I' 



Fig. 971. — -A por- 

 tion of the stem of 

 Bryophyllum caly- 

 cinuntf showing an 

 abundant development 

 of warty emergences, 

 the lenticels ; note also 

 the leaf scars at the 

 nodes (w). 



