140 PLANT LIFE 



same tendency in a still more advanced degree. 

 Only a few of the earliest leaves on the seed- 

 ling exhibit a blade, all the succeeding ones 

 having flattened petioles only. 



More often, however, it is the stem which 

 undergoes modification and develops leaf- 

 like characters. When only certain branches 

 become specialised in this way, as in species 

 of Butcher's Broom (Ruscus), it may require 

 careful examination to detect the cauline 

 nature of the apparent leaves. But the 

 genuine leaves are really present, and although 

 they are reduced to small brown scales, they 

 suffice to indicate the true condition in this 

 as in other extreme examples. 



A still more remarkable modification is 

 seen when the roots assume the functions 

 of green leaves. An instance of this is fur- 

 nished by the genus of epiphytic orchids known 

 as Taeniophyllum (Fig. 18). These orchids, 

 which possess very inconspicuous flowers, are 

 also destitute of foliage leaves. But the 

 function of photosynthesis is discharged by 

 the green, band- or tape-like roots which are 

 appressed to the bark of the trees upon which 

 the plants are growing. In some species 

 the roots are very long, and hang freely from 

 the tree trunk, when their resemblance to 

 narrow strap-shaped leaves becomes addition- 

 ally striking. 



It often happens that new structural modifi- 

 cations adaptations in the making, as it were 

 respond to the influence of the stimulus 



