LECTURE XXVIII. 



THE oosperm by repeated cell-divisions gives rise to the embryo. 

 At first the whole embryo is composed of undifferentiated cells 

 only, quite similar in appearance to one another. Gradually in 

 the base of the embryonic stem and in that of the embryonic 

 root some cells begin to elongate and to form procambial tracts. 

 These in turn become the conducting tracts while the cells round 

 about them become fundamental tissue and cease dividing. A 

 similar differentiation takes place in the cotyledons. In this 

 way the groups of cells still dividing become isolated from one 

 another, and occupy positions at the end of the root and at the 

 tip of the stem ; and the separate growing regions of the plant 

 are established as direct descendants from the oosperm. Hence 

 it is that all the cells and modified combinations of cells, which 

 constitute the mature plant, may be traced back to the subdivisions 

 of this single cell. It may be further noted that the sporogenous 

 cells of the sporophylls are directly traceable to the oosperm. 



Each cell-division in these countless lines of cell-descent all 

 leading back to the one common origin is preceded by the 

 division of the cell's nucleus. Just as each cell results from the 

 division of a pre-existing cell so the nucleus never arises de novo, 

 but each nucleus is the result of the division of a pre-existing 

 one. How this nuclear division is effected it is now of importance 

 to consider. 



The nucleus of a cell in the resting state, i.e. one which is 

 not immediately about to divide, is approximately globular in 

 form. It is limited by an extremely delicate nuclear membrane 

 which encloses a clear nuclear fluid. Suspended in this fluid is 

 a viscid colourless substance, which forms irregular filaments or 

 threads, crossing the fluid and sometimes lying in contact with 

 the membrane and anastomosing with itself, so as to form a 

 very irregular net-work. Adhering to, or embedded in, the net- 

 work are numerous granules of high refractive index, sometimes 

 fairly uniform in size and sometimes variable in size. In many 

 resting nuclei these granules appear to crowd the cavity of the 



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