DIVISION OF SPORE GRANDMOTHER CELLS 215 



while the central cell gives rise to two distinct products, namely, 

 an exterior tissue or tapetum averaging two cell-layers in thick- 

 ness when mature, e and /, and surrounded by this an arch- 

 esporial cell, e, which is soon to give rise by division to a group 

 of cells known as the sporogenous cells or grandmother cells of 

 the spores, f and g. 



It is clear that the tapetal and sporogenous tissues must be 

 classified as reproductive because they serve the purpose of 

 reproduction and no other, the sporogenous cells giving rise to 

 the spores, and the tapetal cells acting as nurse cells to these 

 through all the stages of their development. 



Let us now follow these two groups of cells throughout their 

 career. Cell-division ceases for a time in the sporogenous cells, 

 as at/, and they enter into a state of preparation for a remark- 

 able series of processes that, as we shall soon see, are apparently 

 of fundamental importance to heredity. Meanwhile the walls 

 of the tapetal cells break down, and their protoplasts fuse, form- 

 ing a Plasmodium that is free to circulate throughout the spor- 

 angia! cavity, g. The sporogenous, or grandmother cells of 

 the spores, now become separated, and the tapetal Plasmodium 

 moves in between them, where, completely surrounding them, 

 it is in position to assist in their nutrition to the best possible 

 advantage, g; and in the capacity of nurse the plasmodium con- 

 tinues to function up to the time of the maturity of the spores. 



Division of Spore Grandmother Cells. — Now the grand- 

 mother cells begin to initiate nuclear division. The processes 

 in this' have been followed in detail by Yamanouchi in Osmunda 

 cinnamomea. There at a somewhat later stage than is shown 

 in g. Fig. 122, the nuclear network resolves itself into a thread 

 lying in two parallel strands in close proximity. Fig. 123, A, 

 a and b, and traversing the nuclear cavity in various directions. 

 Later the threads become drawn to one side of the cavity into a 

 compact mass where the parallel members become more closely 

 associated. After this the threads go back to the looser ar- 

 rangement which they had at the beginning. They now shorten 

 and thicken and break by transverse fision into twenty-two 



