FUNCTIONS OF VELAMEN. EXODERMIS 233 



gaseous constituents of the atmosphere (e.g. ammonia), and thus to 

 render them available for the benefit of the shoot. The absorption of 

 ammonia by the velamen has indeed been experimentally demonstrated 

 by Goebel in the case of Odontoglossum Barkeri. The more important 

 part of the problem, however, is that which refers to the condensation 

 of water-vapour. The experiments which have been carried out with 

 regard to this matter by various investigators in greenhouses or in the 

 laboratory have indeed mostly led to negative results. It is, never- 

 theless, quite possible that the water-condensing powers of aerial roots, 

 under natural conditions, are sufficient to confer an appreciable ecological 

 advantage upon plants that possess such organs. In any case the 

 experiments in question were mostly carried out upon severed roots ; 

 but in this way a serious error is introduced, owing to the stoppage 

 of the current of water which normally flows out of the absorbing 

 organ. The problem can only be solved by means of experiments 

 performed upon intact plants growing in their natural surroundings. 



The histological structure of the velamen certainly supports the 

 condensation-theory. The number and delicacy of the fibrous thicken- 

 ings hardly seem consistent with the idea that they merely represent 

 the resistant framework of a capillary apparatus. Both the aforesaid 

 features, however, greatly increase the surface available for condensa- 

 tion, especially in the case of the so-called " fibrous bodies," which in 

 certain instances are associated with the exodermal passage cells ; these 

 fibrous bodies will be more fully discussed later on. 



We have still to consider how the water absorbed or possibly 

 condensed by the velamen is transferred (together with the substances 

 dissolved therein) to the cortex of the root. The cortical paren- 

 chyma is separated from the velamen by a very characteristic inter- 

 mediate layer, the exodermis (or outer endodermis), which in all 

 essential features resembles a typical endodermis (cf. Chap. VII.). The 

 exodermis comprises two different sorts of cells. Those of the first 

 type are more or less elongated ; their outer walls are very often 

 thickened, but never pitted. Among these long cells, shorter, rounded, 

 and almost invariably thin-walled elements, with abundant protoplasmic 

 contents, are interspersed, usually in vertical series (Fig. 89, d). The 

 long cells are the exodermal elements in the strict sense ; they have 

 suberised walls, and are consequently relatively impervious to water. 

 They accordingly serve to prevent the aerial roots from drying up 

 during periods of prolonged drought, the velamen itself being of course 

 incapable of undertaking the protective functions usually performed 

 by a dermal tissue. The small thin-walled cells, on the other hand, 

 act as channels for the inward passage of the water collected by the 

 velamen. 



