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Beer. — Studies in Spore Development . 
fugally to project very slightly above the outer surface of the pollen-grain, 
and to give this a wavy appearance. Before long they project far enough 
beyond the periphery of the exine to give this a distinctly spinous 
character. At about this period the £ rodlets ’ can first be clearly observed 
as minute deeply stained structures lying in the position of the clear space 
or layer noticed at an earlier stage between the outer lamellae of the exine 
and the thickening bands. Both the £ rodlets ’ and the spines now stain 
much more deeply than the rest of the exine, and they are, therefore, very 
clearly distinguished in the sections (Fig. 8). 
At this time the kinoplasmic fibres running between the nuclear mem- 
brane and the £ Hautschicht ’ become obscure, and appear to merge into and 
become lost in the alveolar substance of the cytoplasm. 
In rather older pollen-grains the relation of the parts of the exine 
to one another becomes much clearer. The thickening bands have increased 
greatly in thickness and have become much broader, so that the thin areas 
of the exine between these bands are now reduced to a series of pores 
or narrow channels which represent the exit pores for the future pollen-tubes. 
Moreover, the substance of the bands appears to have undergone a change, 
for these are no longer diffuse and ill defined at their inner margins, but they 
are now sharply marked off from the distinct £ rodlet * layer. The spines 
have grown greatly in size. They are still limited to the angles of the net- 
work of thickening bands, and they are now seen to be spindle-shaped with 
their points projecting for some distance beyond the still very delicate outer 
lamella of the exine and their £ roots 5 occupying the whole thickness 
of the rodlet layers. These £ roots *, moreover, are seen to be double, each 
consisting of two prongs (Figs. 9 a and b). A surface view of the pollen- 
grains at this stage shows that the rodlets are limited to the positions over- 
lying the hexagonally arranged thickening bands, and that they themselves, 
therefore, form a hexagonal figure when viewed from above. This is 
shown clearly in Fig. 18, although this represents an older pollen-grain 
in surface view. 
The protoplast of the pollen-grain, which completely fills the pollen- 
cavity, has meanwhile become much poorer in substance and more vacuo- 
lated than at earlier stages. As the pollen-grain has increased from about 
32 n to about 45 fx in diameter the decrease in protoplasmic density is 
at any rate partly due to its substance being distributed over a larger area, 
but I believe that there is also a real loss of substance by the protoplasm, 
which has contributed some material to the growing membranes. The 
nucleus has only increased very slightly in size ; the average of a number of 
measurements showed only an enlargement of about 2 ^ (from 10 /x of an 
earlier stage to 1 2 /x now). The nuclear reticulum has become somewhat 
coarser and stains more deeply : one, two, or often more rather small nucleoli 
may occur. 
