ii6 



GYMNOSPERMS 



wall and numerous sporogenous cells are produced. The tapetum 

 becomes clearly distinguishable rather late, so that it cannot always 



be determined with certainty 

 whether it is coming from the 

 progeny of the primary wall 

 cell or from that of the primary 

 sporogenous cell. But, what- 

 ever its origin may be, it is in 

 contact with spores which it 

 is to nourish. 



As the spore mother-cells 

 enlarge, the tapetum breaks 

 down and appears as a mass 

 of nucleated protoplasm sur- 

 rounding the growing mother- 

 cells, which absorb not only 

 the tapetum but also the wall 

 cells between the tapetum and 

 the epidermis. The epidermal 

 cells become very much thick- 

 ened, especially at the bottom 

 and along the sides, while re- 

 maining thin at the top, where 

 the cell contents break through 

 and escape (tig. 113). 



The spore mother-cells 

 round off and each divides 

 twice to form four micro- 

 spores, thus bringing the 2X, 

 or sporophyte, generation to 

 a close and initiating the .v, or 

 gametophyte generation. In the first mitosis 12 pairs of chromo- 

 somes are easily distinguished, and in the anaphase of the second 

 division the number 12 is easily counted. 



Fig. 1 10. — Ceratuzamia mexicana: four 

 microsporophylls; a, with sporangia not yet 

 dehisced; b, sporangia at the upper part de- 

 hisced and the pollen still holding together in 

 balls; c, later stage, with nearly all of the 

 pollen shed. The sorus arrangement (mostly 

 threes) is more easily seen in the later stages; 

 X2. — After Chamberlain.'" 



