xix] MACROPODOUS EMBRYOS 249 



of seeds with elaborate and impervious coats, seems to indicate 

 that plants with imperfectly protected embryos have been unable 

 to enter upon aquatic life. Possibly there is a danger of rotting 

 if the contents of the seed are at all freely exposed to the water. 

 If this is so, it may be that an embryo which keeps its reserves 

 inside its own tissues is better adapted for water life than one 

 whose storehouse is outside its own body, even if it is enclosed 

 in a resistant coat; the food is probably more secure from the 

 depredations of Bacteria and from other harmful external in- 

 fluences, if it is incorporated within the cells of the embryo 

 instead of being merely surrounded by the testa. In the opinion 

 of the present writer, Monocotyledons have, in general, re- 

 duced their seed-leaves to a single cylindrical or tubular struc- 

 ture by means of the fusion of the petiolar or sheathing regions 

 and the loss of the blades. They are thus not in a position to 

 store food in the laminae of the cotyledons, as is done, for in- 

 stance, in the case of such Dicotyledons as the Pea or the Bean. 

 The radicles of aquatic seedlings are, as we have already shown, 

 markedly reduced, so a second possible location for food storage 

 is thus eliminated. In this connexion we may recall the fact 

 that, whereas mature Dicotyledons often store food in their 

 tap roots (e.g. Carrot, etc.) this method is unsuitable for Mono- 

 cotyledons, owing to the ephemeral nature of the primary 

 radicle, and they are hence almost wholly restricted to storage 

 in leaf structures, stem structures, or adventitious roots. We 

 are thus left with the fact that if a Monocotyledonous embryo 

 is to store its food in its own body, the only region where this 

 can be conveniently accomplished is the hypocotyl, since both 

 cotyledon and primary root have suffered reduction. From 

 these considerations we may perhaps conclude that the non- 

 endospermic type of seed with a macropodous embryo, whose 

 hypocotyl has become enlarged for food storage, represents a 

 form of Monocotyledonous seed which is particularly well fitted 

 for aquatic life. 



