332 



THE SEED-HABIT 



As regards the former feature, the only essential difference from 

 Selaginella lies in the germination of the microspores in close 

 proximity to the megaspore, for which special devices such as 

 the mucilaginous poUination drop and the pollen chamber are 

 created. Moreover, with the help of the pollen tube some nourish- 

 ment is obtained from the adjacent nucellus. 



The seed may be looked upon as an extreme development 

 of heterospory. Even in Selaginella it was noted that in some 

 species there may be reduction to a single megaspore, which is 

 retained within the sporangium till after fertilisation (cf. p. 319). 



Fig. 191A. — Restoration of the seed (Lagenostoma lomaxi) of Lyginopteris 

 oldhamia, in its glandular cupula. (After Oliver.) 



Such reduction and retention are eminently characteristic of the 

 seed-habit, where dehiscence of the megasporangium is dispensed 

 with. The added protection, furnished by the integument, 

 admits of a change in the character of the sporangium-wall, 

 which becomes a thin-walled tissue ser\-ing as intermediary 

 between the \-ascular sj'stem of the ovule and the developing 

 megaspore and female prothallus. 



The possession of a vascular system, another characteristic 

 of the ovule, facilitates nutrition and the storing up of food- 

 reserves for the young embryo. It is probably in consequence 

 of the increased food-supply that o\'ules and seeds in general 

 attain so large a size as compared with the sporangia of lower 



