THE NUTRITION OF THE ROOT 75 



§ 12. How do agrial roots nourish a plant ? 



This question is of considerable practical importance, now 

 that the cultivation of orchids and aroids has become such a 

 hobby. We must try to ascertain whether the aerial roots 

 are organs by means of which the gardener can improve the 

 nutrition and the development of his plants. This question 

 must be answered in the affirmative ; for the aerial roots con- 

 stitute an absorptive system, and can therefore derive some 

 benefit from an enriched supply of food material, i.e., from 

 manuring. The absorptive portion of the root consists here 

 of a fine but tenacious silvery layer, which surrounds the root 

 like a sheath (velamen), and will be well known to any 

 gardener who has seen the strong aerial roots of a Vanda or 

 Aerides. The genus of Stanhopea lends itself best perhaps 

 to the study of aerial roots. 



If a transverse section be made through the aerial root of 

 a Stanhopea, the central vascular cylinder will be seen sur- 

 rounded as in all roots by the thickened layer, previously 

 described as the endodermis. This layer of cells is surrounded 

 on the outside by the soft cortex, and the latter is enclosed 

 by a layer of thickened cells, which represents the outermost 

 layer, i.e., the epidermis of ordinary roots, the cells of which 

 would form the real absorbing surface, growing out to form the 

 root-hairs. In aerial roots, however, this is not the case ; for 

 this layer does not limit the root on the outside, but is covered 

 in by a tissue (very extensive in Stanhopea, but very delicate 

 in some orchids), and it constitutes, therefore, an outer endo- 

 dermis. The real soft cortex, therefore, is bounded in aerial 

 roots on either side by a layer of endodermis. Outside the 

 outer endodermis we find the root-sheath or velamen. 



The cells of this sheath are fitted together without any 

 intercellular spaces, and form a radially or longitudinally 

 elongated parenchyma, generally strengthened by spiral bands 

 running in the cell-walls. A point of special importance is 

 the fact that the cells have often perforations leading from 

 one cell to another. Such apertures exist not only between 

 the cells in the inner portions of the velamen, but are also 

 found in the walls of the outermost layer. In cells which are 



