xxviii] SUBMERGED POLLINATION 345 



If the view here advocated regarding the nature of the blades 

 of Monocotyledonous leaves be accepted, it forms a particularly- 

 salient instance of the working of the * Law of Loss,' since we 

 have here an instance of a discarded organ (the lamina) being 

 replaced by a modification of another (the petiole) in lieu of 

 being re-acquired. 



St 



xy] 



nbz- 



Fig. 171. A, Sagittaria sagittifolia, L., T.S. lateral vein of lamina, next but one to 

 midrib. B, Sagittana montevidensis, Cham, and Schlecht., small part of T.S. of 

 leaf near margin. The lower of the two bundles belonging to the normal series 

 (n.b._) is irregularly placed, [n.h-^ = bundle of main normal series; i.b. = inverted 

 bundle; xy = xylem; ph = phloem; a.t. = assimilating tissue; st = stomate; 



o.d. = oil duct.) [Arber, A. (1918).] 



The pollination methods of submerged Angiosperms may 

 also possibly be regarded as illustrating the Law of Loss. The 

 ciliation of the male gamete in the great group of the Pterido- 

 phyta from which it is supposed that Flowering Plants are 

 ultimately derived is associated essentially with aquatic fertili- 

 sation; with the adoption of terrestrial life this feature was lost, 

 and is now unknown either in the higher Gymnosperms or the 



