184 MANUAL OF BOTANY 



has a lining of naked or primordial cells. These become sepa- 

 rated from each other and from the cavity of the spore by cell- 

 walls, and the cells so formed divide repeatedly till, in most 

 cases, the part of the embrj^o sac not occupied by the body 

 resulting from the divisions of the zygote becomes filled with a 

 mass of tissue which is the endosperm. In a few cases the 

 endosperm does not completely fill the spore, but leaves a hollow 

 cavit}^ in the centre. 



The young embryo, derived from the zygote in"a manner still 

 to be explained, together with the endosperm and the remains 

 of the microsporangium and its coats, forms the body frequently 

 alluded to above as the seed. The coats or integuments of the 

 macrosporangium generally become hard and form the testa of 

 the seed. 



In some cases the endosperm is not formed by free-cell-for- 

 mation as described above, but by ordinary cell-division, walls 

 being developed after each nuclear fission. 



In some plants this condition of things persists, and the 

 embryo in the ripe seed is embedded in the endosperm. In 

 others the embryo grows vigorously, and in its enlargement 

 absorbs the contents of the endosperm cells. The seed then shows 

 only the embryo surrounded by the testa. Generally the seed 

 leaves or cotyledons are the parts which form the bulk of the 

 embryo in such cases. 



When fertilisation has been effected, and the zygote is 

 formed, it at once commences its further development. There 

 is no pause between the maturity of the gametophyte and the 

 origination of the new sporophyte. A resting period occurs later, 

 when the young plant has attained a certain size and when the 

 endosperm has been formed and perhaps absorbed. The zygote 

 divides into two by a transverse wall, and the two cells so formed, 

 which correspond to the epibasal and hypobasal cells of the 

 Pteridophyta, give rise, as in the latter case, to different structures. 

 The hypobasal segment, which is the one nearest to the micro- 

 pylar end of the embryo sac, develops into a chain of cells known 

 as the sus])ensor, a structure noticed as occurring in a similar 

 manner in the embryogeny of the Lj^copodinge. The suspensor 

 does not become free from the protoplasm of the embryo sac, in 

 which the oosphere was originally placed, and hence attaches the 

 embryo to the embrj'O sac in a way indicated by the i\&\ne suspensor. 

 The suspensor, as a rule, soon perishes, so that only traces of it 

 can be seen in the seed ; sometimes, however, it grows to great 

 length. In the Orchidaceae cases occur where it assumes such a 



