THE LIFE OF A SEED. 
399 
as we can, a statement of those phenomena the interpretation 
of which is now no longer doubtful. Suppose, then, a grain of 
pollen to have fallen on the stigma, and to be there retained by 
the hairs projecting from its surface, the next step is the emission 
of the pollen-tube. This takes place in consequence, apparently, 
of endosmotic action going on between the fluid exudation from 
the stigma and the contents of the pollen cell. The latter, dis- 
tended by the inpouring of additional liquid, bursts, and thus 
liberates the inner lining of the cell, in the form of a cylindrical 
tube. Down between the cells of the style passes the marvel- 
lous conduit, ever lengthening till it reaches the ovules in the 
cavity of the ovary. As the length of the style is sometimes 
very considerable, it was long a matter for surprise that so great 
extension could be manifested by a single cell, but it has recently 
been shown that the lengthening of the pollen-tube is due to 
something more than mere extension, and that an actual growth 
takes place interstitially, varying in degree and in rapidity in 
different plants and in consonance with different circumstances. 
In Tigridia, where the style is very long. Dr. Martin Duncan has 
estimated the rate of growth of the pollen-tube at one inch in 
four hours and a half, though, under favourable conditions of 
heat and moisture, the rate may be accelerated. But with these 
variations we must not occupy ourselves now, but proceed to 
trace the further progress of the seed. Arrived in contact with 
the ovules, whose structure has been before described, the pollen 
tube enters into the foramen at the top of the ovule left by the 
imperfect closing up in that situation of the investments of the 
ovule, and thus comes into contact with the nucleus, and the 
large embryo sac already alluded to. In this latter are sundry 
minute vesicles, some at the top and others at the other ex- 
tremity of the sac. With the latter we need not here trouble 
ourselves, as their existence has not been made out in all cases ; 
where they are found they appear to be transitory only, while 
their purport is at present wholly unknown. 
With the little vesicles in the upper end of the embryo sac, 
generally two or three in number, the case is different. These 
form what botanists call the germinal vesicles, and the conse- 
quence of the application of the end of the pollen-tube to the 
upper end of the embryo sac just over these vesicles is soon seen 
in the rapid growth of one or sometimes two of these latter 
bodies, and which in the course of a short time lengthens into a 
slender cellular thread, at one end of which is formed the em- 
bryo plant, at first a mere globe of cells, but in dicotyledons 
speedily becoming marked out into a radicle or primary root 
at one extremity, while the opposite end divides into the two 
cotyledons. How or why this result follows immediately on 
the contact above described is a profound mystery; there 
