20 6 
Beer. — Studies in Spore Development. 
The pollen-grain continues to increase in size, and its wall grows both 
in surface and in thickness ; in proportion as this growth proceeds the 
protoplast continues to become more vacuolated and poorer in substance, 
although it never contracts away from the pollen-wall, as in Oenothera . In 
pollen-grains which measure 70 /x in diameter the cytoplasm encloses 
a number of large vacuoles, and the nucleus, which now measures about 14 /x 
in diameter, contains one or more nucleoli and a rather scanty reticulum. 
By the time the pollen-grains have reached 80 or 90 \ ix in diameter the cyto- 
plasmic lamellae which separate the large vacuoles from one another have 
become broken down, and the protoplast is reduced to a hollow shell with 
a single huge vacuole occupying its entire centre. This cytoplasmic shell 
consists of little besides a ‘ Hautschicht \ except in the immediate vicinity 
of the nucleus, where some granular cytoplasm still remains (Fig. 10). The 
nucleus is now a flattened body measuring about 20 /x by 10 /x across its 
greatest and least diameters. It enclosed a rather scanty, somewhat faintly 
stained arrangement of threads, and one, two, or more nucleoli of large size. 
The great increase of nucleolar matter is certainly the most striking change 
in the nucleus from its earlier stages ; these large nucleoli may measure 
as much as 8/x across. Not infrequently the interior of the nucleoli has 
a vacuolar appearance. The alteration in the appearance of the nucleus 
which is just beginning to become evident ushers in the process of proto- 
plasmic reconstruction. The cytoplasmic shell is seen to become slightly 
thicker, and the granular cytoplasm, which had been reduced to one small 
area near the nucleus, can now again be observed as a thin layer all round 
the inner surface of the hollow protoplast. In the meanwhile, the nucleus 
has increased in size to as much as 30 \x by 20 /x in its longest and shortest 
diameters. The pollen-grain itself still measures 90 /x across. This nucleus 
enclosed one extremely large nucleolus (rarely two) which on an average 
measures about 12 /x in diameter. The fibrils which traverse the nuclear 
cavity have become much more numerous ; they are finely granular in 
appearance and diffuse, and irregular in outline (Fig. 11). The proto- 
plasmic shell continues to grow in thickness, and before long the single 
central vacuole becomes bridged across by one or two cytoplasmic lamellae 
which divide it up into a few large vacuoles. These become progressively 
smaller as the protoplasmic lamellae grow more massive and more numerous. 
Starch, which hitherto was present only in comparatively small quantities, 
now occurs in great abundance. Granules, or more probably droplets, and 
irregular masses of material which are black in my preparations stained 
with Heidenhain’s haematoxylin also accumulate in the cytoplasm of the 
pollen-grain. The distribution of this dark -staining material in the pollen- 
protoplast is of some interest. It is usually rather densely collected in the 
little peripheral finger-like cytoplasmic processes which project into the 
exit pores of the pollen-wall. From these points the material can be seen 
