72 POTAMOGETONACEAE [ch. v 



fifty fruits of P. natans^ which had been gathered in September, 

 1889, were kept in water at the temperature of the laboratory, 

 none germinated in 1890 or 1 891, six germinated in 1892, and 

 thirty in 1893, i.e. after lying dormant for three years and a 

 half. 



Fig. 46. Potamogeton perfoliaius, L. Transverse section of fruit wall to show air 

 spaces in the outer region of the wall, and also the thick outer cell-wall of outer 

 epidermis (o.e.). The cross-hatching indicates the non-cuticularised part of the 

 wall: only the outermost surface layer, shown in black, is converted into cuticle 

 (c). Chlorophyll grains in epidermis, (x 260.) [A. A.] 



The most striking feature of the Potamogetonaceae, as a 

 family, seems to be the remarkable reduction series shown by 

 the vegetative and reproductive organs the degree of reduc- 

 tion serving in general as a gauge for the degree of completeness 

 with which the aquatic life has been adopted. 



