490 



SECRETORY AND EXCRETORY SYSTEMS 



In a very large number of cases epidermal hydathodes take the shape 

 of multicellular trichomes, which sometimes resemble ordinary tapering 

 hairs, but are more often developed as clavate, capitate or scale-like 

 structures. 



The ternate leaves of Machacrium obhngifolium (Papilionaceae), 

 a Brazilian liane, are covered on both sides with long stiff hairs, 



which act as hydathodes. Each hair 

 consists of a five- to six-celled basal 

 portion or " foot," and of a two-celled 

 "body" (Fig. 195 a). When the 

 hair is fully developed, its long taper- 

 ing terminal cell is devoid of living 

 contents and provided with rather 

 thick lignitied walls ; the partition 

 which separates it from the short 

 subterminal cell (the other cell of 

 the " body ") is oblique, greatly 

 thickened, strongly lignified, and fur- 

 nished with a number of narrow 

 slit-like pits. The lateral walls of 

 the subterminal cell are also very 

 thick, but are cutinised instead of 

 being lignified ; this cell contains a 

 massive protoplast. The base or 

 foot of the hair is composed of four 

 or five very much flattened, disc- 

 shaped cells placed one above the 

 other ; the whole column rests upon 

 a cell which is somewhat less flat- 

 tened. All the basal cells have 

 massive protoplasts. Their trans- 

 verse walls are thin, except for a 



B 



Fig. 194. 



Hydathodes from the adaxial leaf-surface 

 of Plumbago lapathifolia. A. Surface view. 

 B. Vertical section. (Contents of the glandular 

 cells omitted in both figures.) 



thick marginal zone, which becomes 



more and more extensive as one 



passes upwards from the lowermost 



cell ; the lateral walls, on the other hand, are greatly thickened and 



strongly cutinised, except in the case of the lowermost cell, which 



has both its lateral and its basal walls unthickened. 



The actual secretion of water is evidently carried on by the short 

 subterminal cell and by the disc-shaped basal cells, which all contain 

 abundant protoplasm. Since, however, these cells have cutinised lateral 

 walls, the secreted water must escape from the living cells of the gland 

 through the lignified oblique septum that separates the terminal from the 



