26 Dixon . — Fertilization of 
the cells of which are provided with brown cell-walls. These 
cells do not contain nearly so much starch as the cells 
beneath, which have not brown cell-walls and which are 
much softer to cut ; but even while penetrating this upper 
part the pollen-tube is often so completely filled with starch 
that it is very difficult to see the nuclei within it. The 
pollen-tube penetrates the nucellus, exercising a destructive 
influence on the cells in its immediate neighbourhood. These 
cells lose their nuclei and become filled with a brown 
substance ; sometimes one finds a cell completely filled with 
this brown substance except for a clear central space which 
apparently had been previously occupied by the nucleus. 
While in the upper brown tissue, it is not uncommon for the 
pollen-tube to branch two or three times (Fig. 8) : however, 
as soon as the tubes reach the lower limit of this tissue 
(which is about May 1 2) only one branch is continued. The 
future growth downward through the thin-walled tissue, rich 
in starch, is comparatively enormously fast. By the 19th, 
one week later, the pollen-tube had in almost all cases reached 
the top of the endosperm, and in one case found, had penetrated 
between the neck-cells of the archegonium. In pollen-tubes 
at this stage, it is no longer possible to distinguish the nucleus 
of the pollen-tube from that of the stalk-cell. One usually 
finds them both close together in the lower end of the pollen- 
tube. They are apparently beginning to degenerate and are 
already considerably reduced in size and more refractive 
than they appeared in the earlier stages. PL IV, Fig. 12 shows 
a piece of a pollen-tube isolated from the surrounding tissue 
by means of the method, recommended by Belajefif, for making 
preparations of the pollen-tubes of Taxus baccata . Single 
ovules are laid in a mixture consisting of 2 parts of sulphuric 
acid and 100 parts of picric acid to which is added an 
equal volume of water. After being in this mixture for 
twenty- four hours they are carefully washed in several changes 
of water. The pollen-tubes may then be, in the case of Taxus , 
isolated with ease by means of needles. With Pinus 
silvestris , however, this method is seldom successful. 
