IN GYMNOSPERMS 



22 r 



tends to become, and often actually is, a mere means of working up the 

 material stored in the mature spore into gametes and an embryo, and 

 does not possess any functional vegetative system. This is exemplified in 

 Figs. 25, 26 of Selaginel/a, which show the contents of the germinated 

 microspore developed as little more than an antheridium. In Fig. 26 the 

 wall of the spore is ruptured, and the contents are ready to be extruded 

 as numerous spermatozoids. Fig. 24 A shows the megaspore with the 

 female prothallus within it, bear- 

 ing an archegonium. Fertilisation 

 takes place as in Ferns through 

 the medium of water. The ovum 

 after fertilisation forms the embryo 

 which remains for a time embedded 

 in the prothallus: but later it bursts 

 through, and establishes itself as 

 the independent sporophyte. 



In many heterosporous plants 

 the germination takes place after 

 the spores are shed, just as is the 

 case in homosporous plants. But 

 in others germination of the mega- 

 spore may be initiated or even 

 carried through within the spor- 

 angium. This is the case in 

 Selaginella apus (Fig. 24), in 

 which it is evident that, even 

 when the sporangium has not yet 

 opened, the prothallus may be 

 well advanced in the megaspores. 

 Fertilisation may be carried out 



within rhp cnnrsnmnm dft^r itc Median longitudinal section of the ovule of /'/rf 



Sporangium att ItS exce i sa> at time of fertilisation. X 9 . e, embryo-sac 



rnnhire bv meqns nf snprmatn fi " ec ' by l ^ e prothallus; a, the venter; c, the neck of 



an archegonium; a, ovum; , its nucleus! nc, nucellus 

 ZOids derived from spores shed of ovule; /.pollen-grains;*, pollen-tubes ; z, integument. 



(After btrasburger. ) 



from adjoining microsporangia, 



and the embryo may be developed while the megaspore is still within 

 the sporangium. It is no great step from this condition to that seen in 

 the Seed-Plants, in which the megaspore or embryo-sac as it is called 

 in Seed-Plants remains embedded in the tissue of the megasporangium 

 or ovule (Fig. 27). The physiological advantage gained by this step is 

 an important one : there is no longer any need to hurriedly pass the 

 nutritive supplies into the spore before its wall, thickened for protective 

 purposes, stops the process of transfer ; for in the Seed-Plants the wall 

 of the megaspore, no longer needed for protection, remains thin, and the 

 nutrition of the female prothallus can be continued until long after the 

 embryo is initiated within it. 



\ 



