114 



LECTURE Vin. 



water by evaporation; and if true roots here and there come into the same 

 position, as is the case with the aerial roots of epiphytal Orchids and many 

 Aroids, in these also the development of epidermis is more pronounced than in 



ordinary roots. 



If a plant consisting of cell-tissue attains a certain size, and a sharper dif- 

 ferentiation of root and shoot, and of assimilating leaf-like portions in contrast 

 to the shoot-axes results, the demand arises in the plant for channels through 

 which, on the one hand, the matters absorbed by the roots may be conveyed: to 

 the organs of assimilation, and, on the other hand, the organic substance there 

 produced may be conducted to the remaining parts of the body of the plant. 



FIG. 113. — Apex of the shoot of Tradescantia 

 alhifiora rendered transparent in order to show 

 the course of the vascular bundles. 1, 2, 3, the 

 cut-offleaves. (After De Bary.) 



FIG. ii4.~Transverse section through a young inteniode 

 of the shoot axis of Tradescantia albifitfra. b, h vascular 

 bundles. The large cells are parenchyma of the fundamental 

 tissue. (After Ue Bary.) 



This is usually accomplished by means of fibriform arrangements of elongated 

 cells, running from the roots through the shoot-axes to the organs of assimi- 

 lation. In lower stages of organisation, as in the Algae (e.g. many Floridea) 

 and Mosses, this stops short with the differentiation of elongated cells, which differ 

 otherwise but little from the surrounding tissues. In higher stages of organ- 

 isation, on the contrary, i.e. in all vascular plants (the higher Cryptogams and 

 Phanerogams), however, these undergo a very peculiar and complicated develop- 

 ment, as strands of -tissue serving as channels for the sap, and constitute the vascular 

 bundles or fibro-vascular strands. 



