16 DUNCAN, ON THE IMPREGNATION 
spot is to make this extremity of the sac rather angular in 
outline. 
A dark line shows the margin of the embryo-sac when the 
nucleus is examined by transmitted light, and the globular 
shape and refractile contents of the sac throw the sides of the 
nucleus in shade, and its centre in high light (fig. 2). 
The cells of the nucleus are more perfect. 
3. The ovule, when ready for impregnation, is larger than 
in the imperfect flower, the neck of the nucleus is more 
covered by the external coat, and the micropyle is close 
to and touches the “ papillary structure. The same method 
of manipulation suffices to show that the canal of the mi- 
cropyle is open, that the cell-coat of the embryo-sac is perfect, 
and that its anterior extremity blocks up the end of the 
canal. The embryo-sac is now of considerable size ; its con- 
tents are not granular, however, but consist of simply colour- 
less fluid. The appearance of the membrane of the sac is 
now distinctly cellular, the cells being delicate, and, generally 
speaking, they overlap each other at the edges (fig. 3, c). 
The cells. of the external coat of the nucleus are larger, 
more perfect, and firmer. The ovule is now ready for the 
pollen-tube. 
I have never found any cells in the embryo-sae, and it is 
evident that the cells of the membrane of the sac, when seen 
through, cause many illusive appearances in the fluid below 
them. 
I1I.—The pollen-tube issues from the pollen-grain, in- 
sinuates itself between the papille of the stigma, passes into 
the central tissue, and descends the style. The base of the 
style is traversed, and the tube enters the axis of the ovary ; 
the ovules are then only separated from the pollen-tube by the 
tissue of the axis and the “ papillary structure” opposite and 
touching the micropyle. The tube has to deviate from its 
course and pass at right angles to.gain the base or attach- 
ment of the “ papillary structure ” to the axis, and this devia- 
tion is determined by the direction of the vascular bundles, 
which pass from the axis at right angles to reach the hilum 
of the ovules. The pollen-tube cannot traverse the dense 
tissue of the vessels, but is turned outwards and runs along 
them to the base of the placenta, and the “ papillary struc- 
ture,” whose cellular structure is easily pierced, the cells 
making way for and nourishing the tube in its marvellous 
course. Arrived at the margin of the “ papillary structure,” 
the micropyle, being open and pressing against the papille, is 
speedily gained ; the tube now passes along the canal of the 
micropyle and abuts against the anterior and convex end of 
