238 ABSORBING SYSTEM 



a rough surface and a bulbous base ; the hairs are covered throughout by 

 a delicate cuticle, which rests immediately upon the cellulose layers of 

 the wall. The moderately thick waxy covering of the epidermis stops 

 short at the base of each bristle ; hence the hairs are readily wetted, 

 although water will not adhere to the general surface of the epidermis. 

 Except for the bulbous base, the cavity of the hair is occluded by layers 

 of cellulose, which afford a splendid illustration of stratification. The 

 basal portion contains a massive peripheral protoplasmic layer with a 

 nucleus; its thickened lateral and basal walls are abundantly pitted. 

 The adjoining subepidermal mesophyll cells are radially elongated and 

 colourless, and constitute a localised water-storing tissue. If a severed 

 leaf of this plant is allowed to wither and is then repeatedly moistened 

 with water, it fully regains its normal turgidity in a very short space 

 of time. Since the wax-covered epidermal cells cannot be wetted at 

 all, the absorption of water must be entirely effected by the hairs. 

 Under natural conditions, the process probably takes place somewhat 

 as follows. Drops of dew run down the outside of a hair and are 

 absorbed through the outer wall of the swollen base ; after absorption 

 the water diffuses through the pitted walls of the bulb into the adjacent 

 water-storing cells. In Diplotaxis the annular absorbing zone at the 

 base of the hair is not differentiated histologically ; in the somewhat 

 less bulky, but otherwise very similar, absorbing hairs of Hcliotropium 

 liiteum, H. undulatum and H. arbainense, however, the corresponding 

 region is specially thin-walled. The contrast between the thick outer 

 epidermal wall and the thin-walled base of the hair is particularly 

 striking in the first mentioned species (Fig. 92 b). 



Multicellular absorbing hairs often form a felted covering on both 

 sides of the leaf. It is, of course, not every felted hairy coat that is 

 capable of absorbing water. If such a covering is easily wetted and 

 rapidly absorbs drops of water; if further the hairy leaf rapidly recovers 

 its turgidity when immersed or besprinkled in the withered condition ; 

 if, finally, places for the entrance of water through the hairs are 

 indicated by the presence of thin-walled basal cells with abundant 

 protoplasmic contents, it may be safely assumed that the hairs in 

 question serve to some extent for the absorption of water. According 

 to Volkens and E. Gregory, hairs of this type are usually constructed 

 in such a way that one or more thin-walled living basal absorbing 

 elements are surmounted by an elongated cellular filament, which is 

 filled with air, or which has its cell-cavities occluded by massive 

 thickening layers. These dead distal portions of the hair may become 

 interwoven into a felted mass (Petasites albus, P. niveus, Helichrymm 

 graveolens, Salvia argentea, Alfrcdia nivca, Inula Selenium, Atractylis 

 fiava, Ijloga sulcata, etc.), or they may all point in the same direction 



