Remarks on the Impregnation of Plants. 315 
times by three of the grains, and produce as many tubes. The 
pollen of several plants, however, (particularly in the Cucurbitacee,) 
is known to protrude its inner lining from a great number of points; 
and Amici has even seen as many as twenty or thirty incipient pol- 
len-tubes arising from a single grain. Commonly, however, each 
simple and globular grain of pollen produces but a single tube, 
which makes its appearance from whatever portion of the surface 
may chance to be placed in contact with the stigma. This produe- 
tion can hardly be considered as a mere protrusion of the inner lin- 
ing of the grain, since the length commonly attained by the tube is 
greatly disproportionate to the original size of that membrane. It 
should, perhaps, be regarded as a growth of the inner coat, excited 
by the fluid which moistens the stigmatic surface. Yet it is hardly 
probable that this fluid exerts any specific and peculiar agency in the 
production of the pollen-tube, since it has lately been stated* that 
a mixture of sulphuric acid and water causes their production in the 
same manner as the stigmatic surface itself, only with greater promp- 
titude. M. Brongniart has also seen them arise from grains of the 
pollen of Nuphar and some other plants, when floating on water, 
without having been in contact with the stigma. Usually, however, 
water is so rapidly imbibed that the grains suddenly burst so as not 
to admit of their production. The stigma of one plant, moreover, 
is known to excite the same action in the pollen of different species, 
and even of plants belonging to different families. Thus, Dr. Brown 
applied the pollen-mass of a species of Asclepias to the stigma of 
an Orchideous plant, and found that these tubes were produced as 
readily as when Jeft in contact with the stigma of the plant from 
which the pollen-mass was taken. 
The tubes, thus produced in contact with the stigma, penetrate its 
not, however, by means of any peculiar channel, but by 
gliding between the cellules and along the intercellular passages 
which abound in the tissue of the stigma and style. M. Brongniart 
was able to follow them only for a moderate distance into the tissue 
of the style, where he thought that they terminated, and, oe 
at the extremity, discharged the fluid and floating particles of the 
pollen-grain. He conceives that these larger particles pass along 
the intercellular spaces into the placenta, and thence into the mouth 
I have met ciaeith this statement in the article Botany, of the Library of Use- 
fa, Knowledge, but I do not know on what authority it rests. 
