272 STUUCTURAL BOTANY 



We see, then, that in the Fir, the emlryo-sac "becomes 

 filled with tissue before fertilisation. This characteristic 

 is common not only to all Conifers, but to all Gymno- 

 sperms, and is one of the most important distinctions 

 between this Class and all other Flowering Plants. 

 The tissue within the embryo-sac has the same func- 

 tion as the endosperm of the Angiosperms, and is 

 often called by the same name, but, as its origin is so 

 different, it is better to use a distinct term, and call 

 it the prothallus (see Part II., Flowerless Plants). 



After the embryo-sac is filled with tissue, but before 

 it has reached its full size, certain cells of the pro- 

 thallus, at the end towards the micropyle, begin to be 

 distinguished from their neighbours by their larger 

 size and denser protoplasm. These cells are from 

 three to five in number, and are always superficial 

 cells of the prothallus, in contact with the embryo-sac 

 wall at its upper end. 1 



While the neighbouring cells go on dividing as fast 

 as they grow, these few special cells grow enormously 

 without dividing. After a time, however, each of them 

 divides near its upper end by a transverse wall. The 

 upper and smaller daughter-cell thus formed under- 

 goes several further divisions, both transverse and 

 longitudinal. The lower cell remains for a long time 

 undivided, but continues to increase greatly in size. 



The stage which we have now reached is this : the 

 embryo-sac has grown to a great size, and is filled 

 with tissue. At its upper end are from three to five 



1 We always use the word upper, in describing an ovule, for the end 

 towards the micropyle, without reference to the position which the 

 ovule may happen to occupy. 



