‘The Life of the Plant. “187 
worthy that insome Orchideze (Notylia, Oncidium, &c.), the pollination 
of the stigma by pollen-grains from the same flower is actually poisonous, 
having a fatally injurious influence on the whole flower. 
When. pollination has been fully accomplished, the 
pollen-grain, excited by the viscid fluid exuded by the 
stigma, puts out one or more long tubes, the pollen-tubes 
(Fig. 358), which are unicellular and usually simple. 
These penetrate through the con- ; 
ducting tissue of the style, and 
reach the interior of the cavity 
of the ovary in a few hours ; in 
fe B. 
a 
& J 
| Mie 
Nt lh 
Fic. 358.—Pollen-grains putting out their Fic. 359.—Longitudinal section through 
pollen-tube: a Dipsacus Fullonum ; B the uni-ovular ovary of Polygonum 
Cucurbita. Convolr ulus at the time of flowering: 
@ stigma; 6 pollen-grains ; ¢ pollen- 
tube ; @ wall of the ovary; gv the erect orthotropous ovule; se its embryo- -Sac } 
ch chalaza ; two pollen- tubes have penetrated through the conducting tissue of the 
style, one of which has entered the micropyle of the ovule, the other not. (x 40.) 
the case of Colchicum in about twelve. There they come 
into contact with the ovules and attach themselves closely to. 
them. {Fig.+359).. One of the cells of the nucleus of the 
ovule has, in the meantime, grown much larger than the 
rest, and has pressed aside and absorbed the surrounding 
tissue. This is the embryo-sac, in which, even before fer- 
tilisation, two membraneless cells, the eméryontc or germinal 
vesicles,‘ have been formed, which are fertilised by the pollen- 
1 More than two embryonic vesicles sometimes occur, though 
normally in only a few plants, as Cztrus. 
