2 I 1 
Beer — Studies in Spore Development . 
from the nucleus to the ‘ Hautschicht * which is engaged in the organization 
of new cell-wall lamellae. 
6. In older pollen-grains the spines have grown beyond the surface of 
the outer lamella of the exine, and the pollen-grain is now distinctly 
spinous externally. The inner parts or * roots ’ of the spines occupy the 
rodlet layer, and they are double structures each consisting of two prongs. 
These spines, therefore, differ considerably in their development and struc- 
ture from the purely superficial ones of such plants as Althaea or Malva. 
y. As the pollen-grains increase in size the protoplast becomes vacuo- 
lated and relatively poor in substance, until it is finally reduced to a hollow 
shell enclosing one enormous central vacuole. In Ipomoea there is no con- 
traction of the protoplast away from the pollen-wall, as is observable in the 
pollen-grains of Oenothera or in the spores of Isoetes , &c. 
8. The growth of the pollen-protoplast from a hollow shell of cyto- 
plasm to the solid protoplasmic body of the mature pollen-grain is ushered 
in by changes in the appearance of the nucleus. 
This body grows very considerably in size, and there is a relatively 
enormous increase in the amount of nucleolar matter which it contains. 
9. The protoplasm of the older pollen-grain contains a quantity of 
reserve material. Starch, which in earlier stages was scanty, now occurs in 
great abundance. Also a material which blackens with osmic acid, and 
which probably is of a fatty nature, now occurs in some quantity. There is 
reason to believe that this fatty substance is derived from the tapetal cells, 
and that it passes from these through the exit pores of the exine into the 
interior of the pollen-protoplast. 
10. The cytoplasm of the small generative cell which is cut off from 
the large tube cell is almost entirely composed of kinoplasmic fibres. 
11. The tube nucleus is large, irregular, and amoeboid in form. It 
possesses a large nucleolus which is surrounded by a distinct chromatin 
sheath. 
12. An intine develops within the exine. It forms a thin layer over 
the general surface of the exine, but at each exit pore it attains considerable 
thickness and protrudes towards the exterior. Its microchemical reactions 
indicate that it consists of pectic bodies associated with some cellulose. 
13. In older pollen-grains the constitution of the exine is much more 
clearly seen than at earlier stages. It consists peripherally of an outer 
lamella which is very delicate in structure and perforated by countless 
little pores or apertures so that its substance is distributed as a delicate 
reticulum with open meshes. The thickening bands have grown greatly 
both in thickness and in breadth. They now form together a thick layer 
(the ‘ mesospore ’) perforated by the relatively narrow exit pores for the 
pollen-tubes. The outer lamella of the exine dips down slightly into the 
exit pores and covers over the protrusions of the intine at these spots. The 
