234 



ABSORBING SYSTEM 



In the aerial roots of certain Orchids the elements of the velauien 

 which immediately overlie the passage-cells the " cover-cells " of 

 Leitgeb are distinguished by the presence of very remarkable disc- 

 shaped or spherical local thickenings on their inner walls. The 

 structures in question were first observed in the genus Sobralia by 

 Leitgeb, who also demonstrated their fibrous texture. More recently 

 Meinecke has made a detailed study of these peculiar thickenings, 

 which he terms fibrous bodies. A typical fibrous body develops as 

 follows. First a number of delicate fibrous ridges make their appear- 

 ance upon the inside of the cell-wall ; from these arise numerous 

 exceedingly slender rodlets, which at first project at right angles. 

 Soon the fibres and rods all become interwoven into a felted mass, 



Fig. 90. 



Small portion of a T.S. through an aerial root of Sobralia macrantha, showing a 

 passage-cell with a fibrous body attached to its outer wall. 



which may reach a considerable size. Leitgeb supposes that the highly 

 porous body produced in this way first absorbs and retains any water 

 that reaches it, and subsequently transfers the liquid gradually to the 

 underlying passage cell. The author, however, considers it more 

 probable that these fibrous bodies represent minute but highly efficient 

 condensing organs. Their location in contact with the outer walls of 

 the passage-cells agrees quite well with this interpretation. It must, 

 however, be admitted that those who deny the condensing capacity of 

 the velamen are entitled to regard the fibrous bodies as protective 

 plugs serving to retard evaporation from the thin-walled passage-cells, 

 a possibility which was indeed recognised by Leitgeb. In any case 

 the controversy as to the function of these interesting structures can 

 only be settled by experimental investigation. 



In certain cases (e.g. in Angraecum subidatum) the velamen is cast 

 off, when the root has reached a certain age, the exodermis thenceforth 

 filling the place of a normal epidermis. A similar exfoliation of the 



