62 



POTAMOGETONACEAE 



[CH. 



graceful way. A similar undulation is characteristic of Apono- 

 geton ulvaceus. Baker 1 . A curious feature of the leaves of 

 various species, e.g. P. lucens and P. praelongus, is their shining 

 oily surface 2 , which is due to the presence, in the epidermal 

 cells, of large oil drops secreted by special colourless plastids. 

 The non-wettable, slippery surface thus produced may be, it is 

 suggested, a protection against water animals and micro-para- 

 sites. It has also been supposed that the oil may hinder diffu- 

 sion and hence prevent the soluble products of assimilation 

 from being washed out of the leaf. But it seems to the present 

 writer more probable that the oil is a mere by-product of the 

 plant's metabolism; there is no valid reason for making the 

 assumption that it performs any special function in the life- 

 history. 



FIG. 39. Diagrammatic T.S. of stem stele of three species of Potamogeton to show 



reduction and fusion of vascular strands. t lf 7\, * lf traces of next higher leaf; 



*a. T 2> *2> traces of second higher leaf; remaining strands cauline. A, P. pulcher, 



Tuckerm. B, P. natans, L. C, P. crispus, L. [Chrysler, M. A. (1907).] 



Such species as Potamogeton trichoides and P. pectinatus have 

 very narrow submerged leaves which are linear in form and 

 tender and translucent in texture. 



The species belonging to Potamogeton and the allied genus 

 Zannichellia can be arranged, according to the anatomy of their 

 stems and roots, in a reduction series, beginning with the types 

 with floating leaves, whose axes show a complicated internal 

 structure, and ending with entirely submerged, narrow-leaved 

 species, in which the anatomy is reduced to a state of extreme 

 simplicity 3 . But it is uncertain whether this sequence completely 



1 Krause, K. and Engler, A. (1906). 2 Lundstrom, A. N. (1888). 

 3 Schenck, H. (1886) and Raunkiaer, C. (1903). 



