496 



SECRETORY AND EXCRETORY SYSTEMS 



are very typically developed and differ strikingly from the genuine or 

 ventilating stomata (Fig. 199a, b). 



The so-called apical pores, finally, which occur on the leaf-tips of 

 certain [mainly] Monocotyledonous water-plants, represent a peculiar 

 type of hydathode ; the structures in question have been studied 

 more particularly by Sauvageau, Minden, and Weinrowsky. In these 

 instances the mid-rib of the leaf terminates in a little depression 

 (situate usually on the lower surface of the leaf), or in a cavity which 

 is roofed in by the cuticle {Scheuchzeria palustris). Numerous tufts of 

 tracheides project freely into this pit or cavity, nothing in the nature 



Fig. 199. 



Water-pores. A and B. Secale cereale (tip of the cotyledonary sheath). C. Cono- 

 cephalus ovatus. D. Tropacolum majus. (B in surface view ; the rest in vertical 

 section. ) 



of an epithem being present. The depression owes its origin to the 

 degeneration of the guard-cells of water-pores (Callitrichr, Ranunculus 

 aquatilis), or to the disintegration of groups of epidermal cells together 

 with adjoining sub-epidermal layers ; as already stated, the cuticle alone 

 sometimes remains intact at these points. It is worthy of note, that 

 the distal ends of the tracheides often extend right up to the epidermis 

 even before the depression is formed. In Aponogeton distachyum, indeed, 

 Minden has observed that some of the epidermal cells undergo division, 

 become spirally thickened, and thus secondarily assume the characteristics 

 of tracheides. In this way the terminations of the water-conducting 

 system are pushed forward to the actual surface of the plant. 2 



242 



