356 THE SPERMATOPHYTES 



equivalent of the megaspore, or megaspore mother cell, is called 

 the embryo sac. The megasporangium, termed the nucellus, 

 with the embryo sac is contained within one or two protective 

 envelopes, called integuments, and this group of structures con- 

 stitutes the ovule. There is developed within the embryo sac a 

 much-reduced female gametophyte which lives entirely on foods 

 supplied by the sporophyte. The ovule at maturity then con- 

 sists of the embryo sac (megaspore or megaspore mother cell) 

 with the female gametophyte, the nucellus (megasporangium), 

 and the integuments. 



The female gametophytes are quite different in the two great 

 subdivisions of seed plants (gymnosperms and angiosperms). 

 In the first group (gymnosperms) several archegonia are gener- 

 ally formed, each containing a single large egg. In the second 

 group (angiosperms) the female gametophyte is very much re- 

 duced and only one egg is formed. The fertilization of an egg 

 leads at once to the development of an embryo sporophyte 

 within the embryo sac. The embryo sporophyte of the second 

 generation is thus nourished through the ovule by the parent 

 sporophyte of the first. 



The seed is a ripened ovule, that is, an ovule containing an 

 embryo sporophyte so far along in its development that the seed 

 may safely be separated from the parent plant. Morphologically, 

 the seed is composed of tissues representing three generations: 



(1) the integuments and nucellus are of the parent sporophyte; 



(2) the embryo sac contains more or less tissue of gametophyte 

 origin called endosperm l ; (3) an embryo sporophyte of the next 

 generation lies within the embryo sac. 



343. Pollination and fertilization. The retention of the mega- 

 spore (embryo sac) within the megasporangium (nucellus) so 

 that the female gametophyte is contained in the tissues of the 

 sporophyte has resulted in modifications of the structure and 



1 The endosperm of the angiosperm seed has special peculiarities involved 

 with the fertilization of the egg and development of the embryo, as explained 

 in Sees. 362 and 363. 



