466 



HUTCHINSON'S POPULAR BOTANY 



FIG. 571. CAMPYLOTROPUUS OVULE 

 OF PEA. 



(m) Micro;>yle ; (/) funicle 



and a central nucleus (k) ; and in addi- 

 tion to the protoplasm, a number of small 

 cells (m, -, etc.). First and chief is the 

 egg-cell or oosphere * (m). from which, after 

 fertilization, the embryo plant is de- 

 veloped. Its two sister-cells (n %), which 

 co-operate in this work, are known as the 

 synergidce. They are situated somewhat 

 nearer the micropyle' (g) than the oosphere. 

 and constitute with it the " egg appara- 

 tus." The three cells at the opposite end 

 of the embryo sac are the antipodal cells. 

 Soon after their formation each of the 

 antipodal cells becomes invested with a 

 cell-wall, but they appear to play no part 

 in any of the subsequent phenomena, and 

 their use we are not at present able to estimate. 



Watch the process of fertilization. On putting forth its tube the pollen- 

 grain shapes a course down the loose conducting tissue of the style until it 

 reaches the micropyle of the ovule. The tube, it should be explained, is an 

 outgrowth of the inner coat or inline of the pollen-grain through the extine 

 or outer coat (fig. 559), and it bears at the end a reproductive nucleus, which 

 is in fact the male sexual cell or male gamete. Forcing its way down the 

 micropyle, the pollen-tube perforates the embryo sac, and then, opening at 

 the tip, allows the reproductive element which has meanwhile divided to 

 form two male cells to pass out. Thus released, the generative nuclei 

 traverse the synergidse which thereupon shrivel and collapse and, with 



that, one of these male cells enters the 

 egg-cell and fuses with the female nucleus. 

 This is the act of fertilization. 



The cell formed by the fusion of the 

 male and female nuclei is called the oosperm 

 or embryonic cell ; and from this cell (as 

 the latter name suggests) is eventually 

 formed the embryo or baby plant the 

 essential part of the seed. 



Immediately after fertilization endo- 

 sperm begins to form in the embryo sac, 

 either by cell division as in some Dicoty- 

 ledons or by free cell formation as in 

 all Monocotyledons and many Dicoty- 

 ledons. The embryo sac meanwhile 



increases greatly in size and gradually 

 FIG. 572. SPINY FRUIT OF 

 MEGAKRHIZA CAMFORNICA. * Called also the yennsphere and ovum. 



