2O MARGARET C. FERGUSON 



TETRAD-DIVISION. 



The Definitive Arches for -turn. During the period of " rest " 

 preceding the heterotypic division, the microspore-mother-cell 

 increases much in size, its nucleus becoming even larger than 

 an entire cell of the primitive archesporium, as is readily seen 

 by comparing figs, i and 2 with figs. 3 and 4. The walls en- 

 closing the spore-mother-cells thicken considerably, and the 

 cytoplasm assumes a fine, almost granular structure which, 

 under high magnification, resolves itself into a delicate, close 

 reticulum. At this stage, only three or four nucleoli are found 

 within the nucleus, but this reduction in number may be only 

 apparent, for the nucleus has enlarged to such an extent that 

 no one section would be liable to contain as many of these 

 structures as would a section of one of the smaller nuclei of 

 the primitive archesporium. No attempt has been made to de- 

 termine the exact number of nucleoli in the nuclei of the arche- 

 sporium at any time in its history, as it is next to impossible to 

 trace accurately the sections in the series of any given cell when 

 each anther contains hundreds of archesporial cells all of which 

 are practically alike in form, structure and staining capacity. 



As the nucleus of a pollen-mother-ceil enlarges, its reticu- 

 lum becomes more open, the threads of the net gradually in- 

 crease in thickness, the net-knots or karyosomes become more 

 or less prominent, and numerous smaller granules are distrib- 

 uted irregularly upon the linin. Many cross-threads are with- 

 drawn but no true spireme is formed at this time (fig. 3). The 

 thickening of the threads is more prominent in Pinus Strobus 

 than in the other species, the net-knots are more conspicuous, 

 and a somewhat imperfect spireme arises, although here, too, 

 many anastomosing threads still persist (fig. 4). A remarkable 

 change has taken place in the attitude of the different elements 

 of the cell towards stains. When the microspore-mother-cells 

 are first formed both cytoplasm and nuclear net stain more or 

 less diffusely with gentian-violet as in the primitive arche- 

 sporium, but, as growth proceeds, the cytoplasm ceases to react 

 to chromatin dyes and takes the orange G with avidity. The 

 nucleoli are colored far less deeply with the gentian-violet than 



