2o Trates of Unity in Plants. 



rounded form, and in the tendency to cling to other 

 bodies. In the more unusual forms, moreover, as in the 

 Asplenium rhizophyllum, may be found the direct realiz- 

 ation of this double nature : for here the tendril, which 

 is the direct prolongation of the median nervure of the 

 frond, grows in a downward direction, and ends by bury- 

 ing itself in the earth, and so becoming a true root. 



And as with the tendril so with the aerial root. The 

 leaf-nature of this organ is apparent in many cases, and 

 nowhere more so than in that of orchidaceous plants. 

 In the Vanda teres, for example, the aerial roots, and 

 the representatives of the leaves, exist as simple, rounded, 

 and elongated soft tendril-like processes of a green 

 colour, between which the mutual resemblance is such 

 that one might readily be mistaken for the other. In 

 the screw pine (Pandanus], on the other hand, the 

 history of the growth of the plant shows plainly enough 

 that there is no essential difference between the aerial 

 and the common roots : for here the original true roots 

 (in consequence partly of the pressure caused by the 

 growth of the aerial roots) perish presently, and, con- 

 temporaneously with this change, the latter roots increase 

 in size, sink into the ground, and eventually take upon 

 themselves the functions of their predecessors, the true 

 roots, In due time, these aerial roots, which have 

 become ordinary roots, perish like the original roots, and 

 are replaced by other aerial roots ; and so it is that by 

 the carrying on of this process of dying at the centre and 

 growing from an outer rim which is continually widen- 

 ing and rising higher the Pandanus, when full grown, 

 presents the singular spectacle of a tree raised up upon a 

 circlet of stilt-like roots, with a cavity under it large 

 enough to serve as a shelter for animals of considerable 

 size. The case is plain enough. The aerial root is a 



