THE EMBRYOLOGY OF CARPINUS BETULUS, 39 
We will now proceed to the processes occurring in the ovule after the pollen-tube has 
reached the embryo-sac. Of these we have had under investigation about 300 slides. 
Staining with Ehrlich's hematoxylin after fixing with absolute aleohol was found to 
show up the pollen-tubes with the surprising vividness remarked by Treub in his work 
on the pollen-tubes in Casuarina. Little trace has been found of any resting condition 
of the pollen-tube, so that no fresh knowledge has been cbtained as to its fate during 
the period immediately subsequent to pollination, though tubes have been found 
branching freely in the tissue below the styles. The tube occasionally reaches the 
nucellus before the embryo-sac is mature, and before its full complement of nuclei have 
been formed. Such a premature arrival gives rise to more or less branching and coiling 
of the tube around the embryo-sacs (Pl. 6. fig. 2). This also has been observed 
to occur in older ovules, where the embryo-sacs have for some reason aborted. 
Belated pollen-tubes also occur running up fresh and strong long after fertilization 
has been accomplished. 
The course of the pollen-tube in the raphe varies considerably. Sometimes it travels 
in the vascular bundle of the raphe, sometimes quite close to the nucellar wall, and 
courses intermediate between these two frequently occur. 
In the chalazal region the pollen-tube often becomes much swollen and coiled, though 
not by any means invariably so. There seems to be no doubt that it enters the embryo- 
sac, as a rule, at the base of the caecum: occasionally, however, it is found to enter at 
the side, at some little distance from the base (figs. 7 & 9). 
It may branch in the raphe, at the chalaza, or on its way up to the embryo-sac, but 
most commonly it remains unbranched. It is not by any means the most usual course 
for the tube to run straight up through the middle of a cecum to the egg-cell at the 
micropylar end of the sac. It may follow the wall of the cecum during the earlier part 
of its course, and often crosses the sac, but its path invariably brings it into more or less 
close connection with at least one definitive nucleus, and often with several, as it thread 
its way among the ceca (fig. 11). 
It is exceedingly difficult to distinguish the generative nuclei in the protoplasmic 
contents of the tube, but in some cases they have been found, both before and after it 
enters the chalaza. In the section drawn in fig. 4 the two generative nuclei are clearly 
seen preceded by the vegetative nucleus. The former are separated by an interval of 
about 10 lengths from each other, while the latter is at some little distance from the tip. 
of the tube, whieh has not yet reached the egz-cell. 
The pollen-tube liberates one of these male gametes, probably the one furthest from 
the tip, either just after it enters the chalazal end of the cæcum, or as it passes the 
definitive nucleus. A plug seems frequently to be formed in the tube when this is 
about to occur (fig. 8, a). The gamete appears usually to escape by means of a short 
branch or spur, though in some cases in which the tube passes close to the definitive 
nucleus it may simply be emitted through an opening produced by a local softening of 
the tube-wall. The tube is often considerably swollen at the place where it pierces the- 
embryo-sae wall, and it seems probable that the gamete sometimes escapes at this 
point (fig. 7). 
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