74 AERIAL ROOTS 



a feature doubtless merely related to their growth in the light, 

 since ordinary terrestrial roots [e.g. those of the Pea) will often 

 become green when exposed to illumination. In extreme cases 

 [e.g. Tmniophyllum) the entire carbon dioxide assimilation is 

 carried out by means of the aerial roots, which assume a leaf -like 

 appearance, whilst the true leaves are reduced to mere scales. 

 The absorption of water in such roots is accomplished with the 

 aid of a tissue known as the velamen (Fig. 33, F.), formed by 

 a remarkable development of the epidermis. The dermatogen 

 usually divides to form several, or many, layers of cells, which 

 ultimately lose their living contents and often exhibit a peculiar 

 spiral or reticulate thickening. Moreover, large holes frequently 



Fig. 32. — Diagrams of a normal absorbing root (B) and of a prop-root (A) 

 of the lUaize (Zea mais). Tlie sclerenchymatous tissues are indicated 

 by shading. V ., vessels. 



develop in the walls of many of the cells, as a result of which 

 any moisture faUing or condensing on the surface is rapidly 

 absorbed. The cxodermis (Fig. t,^,, Ex.), situated at the inner 

 edge of the vciamen, is interrupted by thin-wallcd passage-cells 

 (P.) for the inward transference of the water absorbed. In 

 dry weather air fills the cells of the vciamen, so that they 

 appear white and opaque ; but when occupied by moisture they 

 become translucent and the green colour of the cells beneath 

 is visible. 



The ccUs in the central region of the root-cap in many roots 

 contain numerous large starch-grains, which readily move from 

 one part of the cell to another, when the direction of the root 

 is altered. Thus, in the normal erect position they lie against 

 the cytoplasm hnmg the lower walls, but if the root be placed 



