178 NATURAL SCIENCE. Sept.. 



ingly transitory and peels off almost simultaneously with the root-cap 

 cells. The last reason for believing that the velamen is adaptive is 

 afforded by its re-appearance in Epiphytes belonging to the widely- 

 separated family, the Araceae. Thus Epiphytes acquired their vela- 

 men after they had deserted their terrestrial mode of life. 



When young, the aerial roots of orchids usually contain chloro- 

 phyll ; but their assimilatory function is insignificant. However, 

 in ^ranthus and in Taeniophyllum the leaves are reduced to scales, 

 while the roots are large and green. The plant consists almost 

 solely of root-system. In the roots of ^vanthus, Schimper dis- 

 covered that certain regions of the velamen are impermeable lo 

 water, but are easily penetrated by air. These regions obviously 

 function as stomata do in leaves. In A . fasciola the leaf-like appear- 

 ance of the roots is heightened by their dorsi-ventral structure. 



Class I. — Group ii. 



Members of the second group of Epiphytes possess roots of two 

 sorts — they are hence heterorhizal, and exhibit an advance on the 

 first class. When the seed germinates on the host-plant, some roots 

 are produced which clasp the host like tendrils. These are purely 

 organs of attachment. The plant also produces long nutritive roots 

 which descend rapidly to the ground. 



Such roots are possessed by Clusia and Ficns, among Dicotyledons: 

 and by species of Philodendron, Anthurimn, and Carliidovica, among 

 Monocotyledons. 



The attaching roots are negatively heliotropic, but have no 

 marked geotropic properties. They have a slow, definite growth, 

 and coil closely round their support, giving off numerous root-hairs. 

 They have no absorptive function. In them the xylem consists 

 mainly of thick- walled lignified fibres with relatively few and narrow 

 conducting constituents. 



The nutritive roots, in some cases, possess negatively heliotropic 

 characters, but they are always positively geotropic to an eminent 

 degree. Their growth is rapid and indefinite, some roots attaining a 

 length of more than a hundred feet. Till reaching the soil they 

 remain thin and unbranched, but on dipping into the soil they give 

 off numerous subterranean lateral roots. The nutritive materials 

 from the soil have to travel over long distances by way of these roots. 

 The conducting constituents of the xylem are accordingly numerous and 

 prominent, whereas the fibres are feebly developed. The necessary 

 amount of rigidity is ensured by the formation of peripheral sheaths 

 of sclerenchyma or coUenchyma. 



Clusia and Ficus frequently "strangle" their hosts by investing 

 their trunks with a fenestrated cylinder formed of the fused roots 

 which embrace the trees on which these Epiphytes originally lodged. 



Class J. — Group Hi, 

 The members of the third group possess arrangements fcr the 



