136 Agnes Robertson. 
pollen-sacs which were before lying radially, closely appressed to the 
sporophyll stalk, now turn through an angle of ninety degrees or 
more and come to occupy a position tangential to the cone, and at 
right angles to the sporophyll stalk. The}'' each open by a slit 
along the under side (Figs. 5a, b, c). In the ripe cone the cells 
forming the wall-layer are stiffened by fine rib-like thickenings 
(Figs. 15a and 15b). 
III. —The Development of the Microspores. 
Sections of young male cones collected in November at the 
stage when they are three or four millimetres long (including the 
bracts), shew a number of pollen mother-cells already formed, and 
cell-divisions still taking place (Figs. 1 and 8). The nuclei are large 
and nearly fill the cells. As a rule they contain two nucleoli, each 
surrounded by a clear space. Starch is absent from the pollen 
mother-cells, but present in the cells of the sporophyll. The same 
distribution of starch apparently persists throughout the winter, as 
it is still found in cones collected on March 4th. In cones of this 
date a certain number of the sporogenous cells are in a state of 
division though these are fewer than in the November material. 
The structure of the mother-cell nuclei shews no alteration. The 
tapetum generally consists of one layer of somewhat flattened cells. 
By March 19th a considerable change has taken place; the mother- 
cells have increased in size and become packed with starch, and in most 
of the cones the nuclei have passed into a condition of synapsis in pre¬ 
paration for the reduction-division. The chromatin collects into a 
deeply staining ball at one side of the nuclear cavity, and as this 
takes place simultaneously in all the cells, sections at such a stage 
presentavery striking appearance (Figs. 2 and 9). A single nucleolus 
can in some cases be demonstrated in the interior of the chromatin 
mass on treatment with iodine. (Chamberlain 1 mentions that in the 
case of Pinus Laricio he found the pollen mother-cells in the 
synapsis condition in material collected on April 4th). Nearly three 
weeks later the division into tetrads takes place. Material collected 
on April 8th shews the mother-cell nuclei already divided into two 
or four, and wall-formation taking place (Figs. 3 and 10). Some¬ 
times the spindles of the two divisions could be recognised ; but 
unfortunately the material was badly fixed, and it was impossible to 
estimate the number of chromosomes. However the contrast 
between the thick V-shaped chromosomes of the heterotype 
1 C. Chamberlain. “ Winter Characters of Certain Sporangia.” 
Bot. Gaz., Vol. XXV., 1898, p. 125. 
