l8 MARGARET C. FERGUSON 



entiated in the spring, when the mother-cells are in the early 

 prophase of the heterotypic division. The mitoses leading to 

 development of this layer have not been studied, but there are 

 indications that it is formed from the outer layer of the sporog- 

 enous tissue rather than, as usuall}'" described, from the inner 

 layer of wall-cells. The microsporangium-wall, after the 

 appearance of the tapetum, is composed, as before, of three 

 or four layers of cells ; furthermore, the tapetum is always inti- 

 mately associated with the sporogenous tissue, while it is fre 

 quently found separated from the wall of the pollen-sac, probably 

 as a result of imperfect fixation. The question as to the origin 

 of this tissue in Pimis must, however, await further investiga- 

 tion. During the later stages of division in the pollen-mother- 

 cells, the tapetal cells increase much in size, their cytoplasm 

 becomes very dense and each cell comes to have from one to 

 three nuclei which have been observed in all stages of fusion. 

 Karyokinetic figures have been frequently noted in the tapetal 

 cells indicating that the nuclei of these cells divide mitotically, 

 and the division conforms to the ordinary or typical method of 

 mitosis. When the young microspores become free, these cells 

 have attained to their greatest size, and show a diffuse reaction 

 to stains. From this time they gradually diminish in size and 

 finally disappear altogether. The nutritive function of this 

 tissue is too well understood to require discussion here. 



The Primitive Archesporiuni. — With the exception of Pintis 

 Strohus, the primitive archesporium is clearly differentiated 

 in the autumn, but the mother-cells of the microspore do not 

 arise until the latter part of April, and in Piims Strobus not until 

 about three weeks later. 



In the younger stages of development, a superficial study 

 shows no sharp demarcation between archesporium and wall, 

 but a careful examination reveals certain differences by which 

 the two can always be distinguished. The cells of the arche- 

 sporium are larger, have larger nuclei, and denser cytoplasm 

 than those of the wall. They are also polyhedral in outline 

 while the wall-cells are somewhat tabular from the first, though 

 not so markedly so as at a later period. During the winter, the 

 nucleus of a primitive archesporal cell contains several nucleo- 



