240 ABSORBING SYSTEM 



the hairy covering of which was found to absorb water with avidity 

 showed an increase of weight of 10 per cent., after being immersed for 

 twenty-four hours (the same precautions being observed as in the previous 

 case). It may be noted in passing that particular care must be taken, in 

 experiments of this kind, not to carry out the second determination of 

 weight, until the water which is retained by capillarity in the interstices 

 between the hairs has evaporated. 



De Bary long ago remarked upon the frequent occurrence of 

 embedded, sessile or shortly stalked, capitate or clavate hairs of a non- 

 glandular character. These hairs appear in many cases to represent 

 absorbing organs of a special type, the actual absorption being carried 

 out by the one- or more-celled capitate or clavate distal end of the 

 hair, which is provided with abundant protoplasmic contents. The 

 outer wall of the stalk cell is often furnished with a strongly cutinised 

 annular thickened zone, serving to keep the diameter of the channel 

 through which water passes from the hair into the mesophyll constant, 

 even in the wilted condition of the leaf. The one- or several-celled 

 basal portion, finally, is often enlarged, in order that the greatest 

 possible number of photosynthetic cells may be brought into direct 

 communication with this part of the absorbing organ. In Pcperomia 

 scandens the basal cell is prolonged inwards, and often projects far down 

 into the water-tissue. 



"Where organs of this kind occur on leaves which are found by 

 experiment to be capable of absorbing large quantities of water in a 

 short space of time, where further the quick penetration of coloured 

 solutions into the cavity of the hair and the rapidity with which 

 plasmolysis takes place indicate that the outer wall, though cutinised, 

 is nevertheless comparatively pervious, it is fairly safe to conclude that 

 the hairs in question represent water-absorbing organs. The absorption 

 of thin films of water is further facilitated by the circumstance that 

 such hairs are generally inserted below the general level of the 

 epidermis, or else are brought distally into close contact with the 

 epidermal surface, owing to curvature of the stalk-cell. As examples 

 we may mention the capitate hairs of Syringa vulgaris and the clavate 

 hairs of Vaccinium Vitis Idaea (the latter ace. to Lundstrom). Both 

 these instances, however, require further investigation. As a matter of 

 fact, our knowledge of absorbing hairs of this type is altogether 

 insufficient ; in many cases they certainly also serve as water-secreting 

 organs (cf. Chap. X.). 



The most highly organised foliar water-absorbing organs are the 

 absorbing scale-hairs of the Bromeliaceae, our knowledge of which we 

 owe chiefly to the researches of A. F. W. Schimper. Among the forms 

 that have their leaves arranged iu rosettes, the absorbing scales are 



