280 A TEXTBOOK OF BOTANY [Ch. VI, 4 



one fundamental feature, despite their diversity in other 

 respects. 



So certain, indeed, is uhe identity of the sex cells through- 

 out animals and plants that of late some botanists have 

 adopted from zoology the terms egg and spermatozoid, 

 and use them instead of the older terms egg cell and sperm 

 cell. The latter words are retained in this book, because while 

 indicating sufficiently the morphological relations, they 

 are more closely associated with the study of plants. Egg 

 cells and sperm cells are called collectively germ cells. 



We examine first the cytological basis of the fusion of the 

 germ cells. The student will recall that the protoplasm 

 in a living cell is differentiated into the cytoplasm, which is 

 clearly the constructing part, the plastids, which have 

 chemical functions, and the nucleus which seems to act as 

 a control center of the work and development of the cell 

 (page 41, Fig. 16). Now as to the nucleus, its essential 

 part is its chromatin, which lies embedded in the gelatinous 

 protoplasm constituting most of its substance. This chro- 

 matin is a distinct chemical substance, colorless in the 

 living cell, but possessing a remarkable power to absorb 

 colored stains (whence of course its name) ; and it ordinarily 

 exists in fine granules strung together in threads which are 

 much intertangled (Fig. 193, 1). This is its usual, or resting, 

 state, but when the cells are about to divide, in reproduction 

 or growth, the chromatin threads disentangle themselves 

 and collect into definite elongated bodies called chromo- 

 somes (Fig. 193, 7). The number thereof is always definite 

 for each kind of plant or animal, though varying widely 

 for different kinds, — all the way, in fact, from two to more 

 than one hundred. Whatever the number, however, there 

 is good reason to believe that they keep their identity even 

 in the resting state, so that the chromosomes which come 

 out of the seeming tangle are identically the same ones which 

 went into it. Now in the fusion of the nuclei of the germ 

 cells, the sperm nucleus passes over its chromosomes to the 



