630 LECTURE XXll. 



Isoetes) it is only partially exposed by the rupture of the 

 coats of the spore, and in the Phanerogams, where it is 

 termed the endosperm, it remains permanently and completely 

 enclosed by the spore (embryo-sac). Thus the oophore-gene- 

 ration is represented, in the life-history of these heterosporous 

 plants, by two sexual individuals, the male and the female 

 prothallia, which are respectively developed from a micro- 

 spore and a macrospore. The oospore developes into the 

 sporophore which is the highly differentiated plant bearing the 

 sporangia and spores. 



We will now digress for a moment to consider the for- 

 mation of the seed of Phanerogams. It has been already 

 mentioned in the course of this lecture (p. 603), that the macro- 

 spore (embryo-sac) of these plants is peculiar in that it is 

 not liberated from the sporangium (ovule) in which it is 

 produced, and we have just learned that it not only remains 

 within the sporangium but that it also germinates there. 

 Further, the sporangium itself remains attached to the parent- 

 plant. Hence we have this peculiar state of things, that 

 the asexual generation (the plant) bears macrosporangia in 

 which, at a certain time, the sexual generation (the female 

 prothallium or endosperm) is enclosed. But this is not 

 all. When the oosphere, which belongs to the endosperm, is 

 fertilised, it developes into the embryo, the whole being 

 still enclosed by the sporangial (ovular) tissue. The de- 

 velopment of the embryo proceeds up to a certain point, and 

 is then arrested. When this point is reached the conversion 

 of the ovule into the seed is complete. 



According to the structure of the seed, we find that two 

 or more successive generations are represented by its various 

 parts. Thus, in an "albuminous" seed we trace the presence 

 of tissues belonging to three generations; 



Integuments, and peri- | = tissue of the parent- 

 sperm, if present J sporophore 



Endosperm = tissue of the prothallium 



(oophore) 

 Embryo = the new sporophore. 



