Agnes Robertson. 
138 
Taxodium' brings out clearly the contrast between the flattened 
homogeneous looking generative nucleus, and the larger more openly 
constructed tube nucleus. Similarly in his paper on Podocarpus z 
he mentions that the generative nucleus is always denser than the 
other free nuclei of the grain; and from his figures it appears that 
it passes through a flattened period from which it again recovers. 
The exine cannot be distinguished from the intine until the period 
when the generative nucleus retires to the side of the cell. More 
advanced cones gathered on the same day were shedding their 
pollen-grains. These are angular and tetrahedral and shew three 
ribs radiating from a point (Fig. 13). On adding water the grains 
swell with extraordinary rapidity, and in this swelling both cell-body 
and intine take part. The exine bursts and the grain emerges from 
it. In the only case in which I actually saw the grain in the act of 
leaving the exine, it shot out like an orange pip nipped between the 
fingers. Among his observations on Sequoia sempervirens 3 Lawson 
records that the casting off of the outer wall of the pollen-grains 
takes place with considerable force. The swelling of the intine is 
most conspicuous in the equatorial region, and a little papilla is 
sometimes observed at the end adjacent to the larger cell (Fig. 14). 
In these ripe pollen-grains the two nuclei are separated by a parti¬ 
tion dividing the cell into two very unequal parts, of which the larger 
is probably the tube-cell and the other the generative cell. The wall 
between is thin, and either straight or convex towards the larger cell. 
The mature pollen-grains contain little or no starch, whereas a few 
uni-nucleate grains among them, whose intines are much thinner, 
still contain a good deal of starch. As the loss of starch and the 
thickening of the intine take place synchronously it seems reasonable 
to infer that the starch is used up to form the thickened wall. If I 
may hazard a conjecture as to the use of the thick intine, I should 
like to suggest that by its mucilaginousness it may anchor the grain 
to the tip of the nucellus, and also it may conceivably serve as a 
store of reserve carbohydrate, upon which the germinating pollen- 
grain can draw when it is manufacturing its long cellulose tube. 
1 W. C. Coker. On the Gametophytes and Embryo of Taxodium. 
Bot. Gaz., Vol. XXXIV., 1903, pp. 1 and 114. 
1 W. C. Coker. Notes on the Gametophytes and Embryo of 
Podocarpus. Bot. Gaz., Vol. XXXIII., 1902, p. 89. 
3 A. A. Lawson. The Gametophytes, Archegonia, Fertilisation, 
and Embryo of Sequoia sempervirens. Annals of Botany, 
Vol. XVIII., 1904, p. 1. 
