232 



ABSORBING SYSTEM 



associated to form broad ribbons or belts. Eeticulate thickenings are 

 less common than those of the spiral type, but occur for example in 

 Dendrocolla teres and Vanela furva. .Rarely the walls are uniformly 

 thickened and provided with more or less numerous pits {Angraeewn 

 subaJntinn) ; in a few instances, finally, the cell- wall remains thin 

 throughout (THchotosia ferox). All these various forms of thickening 

 sometimes occur side by side in the same velamen, while the types of 



thickening may differ even on differ- 

 ent sides of one and the same cell 

 (Rcnanthera matutinei). Between 

 the thickening fibres the cell-walls 

 are often perforated by holes ; these 

 perforations are not confined to the 

 septa between adjoining cells, but 

 occur also in the outer walls of 

 the superficial layer. The pre- 

 sence of the holes can be simply 

 demonstrated by dipping a [dry] 

 aerial root in water, when the 

 liquid is found to be rapidly ab- 

 sorbed. The frequent occurrence 

 of minute Algae {Protococcus, Ra- 

 phielium, etc.) in the interior of 

 fig. S9. velamen-cells likewise indicates the 



A. Vclamen-cell from an aerial root of Stanliopea presence of perforations. Leitgeb, 



oculata, showing the thickening fibres of the wall. , 



X480. B. Small portion of the exodermis in surface finally, has pi'OVed the existence 



view (from a tangential L.S. through the root); ', d, . 



passage-cells. C. Part of a radial L.S. through the 01 holes 111 the Cell-Walls by 



root ; e, exodermis ; d, d, }>assage-cells. ... . . . 



injection of the velamen with an 

 emulsion of vermilion and water. In their adult condition the velamen 

 cells almost invariably contain nothing but air ; to this fact is due the 

 silvery appearance of such aerial roots. 



From a physiological stand-point, the velamen must be regarded 

 as the absorbing tissue of the aerial root, a fact recognised long ago by 

 Schleiden and Unger. When it is recollected that the walls of the 

 velamen-cells are perforated at a number of points, and that their air- 

 containing cavities communicate freely with the outer atmosphere, one 

 can readily comprehend why this spongy tissue greedily absorbs rain- 

 water and dew. This notion of the capillary action of the velamen was 

 brought into prominence first of all by Duchartre, and subsequently 

 by Schimper and Goebel. While probably correct, it is by no means 

 incompatible with the opinion of the earlier observers (such as 

 Schleiden, Unger, Chatin, and Leitgeb), who believed that the spongy 

 texture of the velamen enables it to condense water-vapour and other 



