REPRODUCTIVE CELLS. 137 



nucleus derived from the microspore (see p. 13 2) with one of the 

 oosplieres contained in the archegonium at the upper pole of the 

 prothallium in the macrospore (embryo-sac). The pollen-tube, 

 derived from the further growth of the vegetative cell of the 

 microspore, grows through the tissue at the apex of the nucellus 

 of the ovule until its tip rests upon the ui)per cells of the 

 prothallium at the top of the embryo-sac in the vicinity of the 

 canal of an archegonium. The nucleus of the generative cell 

 now travels to the tip of the pollen-tube, and, lying in a mass of 

 cytoplasm existing there, divides into two (see Fig. OS, e). One 

 of these nuclei (the so-called male pronucleus) penetrates the 

 canal of an archegonium, being probably attracted by a substance 

 (enzymic in nature) secreted by the neck canal cells, and enters 

 the oosphere, where it lies for a short time close to the nucleus of 

 the oosphere (the so-called female pro-nucleus). Fusion of these 

 two nuclei now occurs, and the resulting nucleus travels to the 

 lower end of the fertilised oosphere, or oospore, as it is now called. 



This nucleus, and the cytoplasm at the lower end, now divide, 

 giving rise to two cells devoid of cell-walls. In each of the 

 resulting cells a division at right angles to the direction of the 

 former one arises, so that four cells lying in the same plane are 

 produced. Each of these four cells divides again twice, so that, 

 finally, there are present at the lower pole of the oospore four 

 rows of cells, there being three cells in each vertical row. The 

 lowest cell of each of these tiers is a potential embryonal cell 

 (pro-embryo), the middle cell in each row is the suspensor-cell, 

 and the upper cell later on di^ippears, or forms, with the remains 

 of the oospore above, pabulum for the lower cells (see Fig. 105). 



The suspensor - cells soon elongate greatly and push the 

 embryonal cells before them deep into the prothallium. Each of 

 the embryonal cells then divides into two cells by a somewhat 

 oblique wall, the uppermost being the epibasal cell and the 

 lower one the hypobasal cell. These two cells are divided again 

 by a wall at right angles to the first, and the next division 

 results in the formation of an octant, from the segments of 

 which the rudimentary organs are produced somewhat after the 

 same manner as in Angiosperms, subseciuent growth proceeding 

 from a primary apical cell, which forms the apical tissues. 



In the above process, then, four embryonal cells are formed, 

 but in reality only one becomes a fully developed embryo. 



