114 
POPULAR SCIENCE REVIEW. 
Fritzsche, “ Beitrage zur Kenntniss cles Pollen,” 1832; and 
Mohl, 66 Beitrage zur Anatomie und Physiologie der Grewachse,” 
1834; the first of these being the only one bearing at all a 
recent date. There is, besides, a magnificent series of unpub- 
lished drawings by Bauer in the British Museum. 
The size and form of the pollen-grain is, within very narrow 
limits, uniform in the same species ; and an adaptation may 
frequently be observed to the normal mode of fertilisation of 
the species. In the pollen-grains of which drawings will be found 
on Plate CXIX., the size varies from less than - 5 -- 0 V 0 °f an i nc h 
in diameter, in the case of Hoteia japonica (fig. 38), to nearly 
in that of Gobcea scandens (fig. 54). In relation to their 
mode of pollination, flowers may be divided into two classes, the 
anemophilous,” in which the wind, and the “ entomophilous,” 
in which insects are the main agent. The former are almost 
without exception flowers without beauty of form, colour, or 
scent ; to the latter belong all plants v T ith conspicuous, brightly- 
coloured, or sweetly-scented flowers, and, as Darwin has pointed 
out, all with irregular corolla. Between these two classes of 
flowers there is a marked difference in the external form of their 
pollen-grains. In the former case the object is to enable them 
to be carried easily by the wind to reach the female flowers (for 
they are very commonly unisexual), or the pistil in some other 
flower. To effect this purpose the pollen is always very dry 
and dusty, the grains not very large, usually nearly or quite 
spherical, and never spiny nor marked with conspicuous furrows or 
protuberances. The following pollen-grains drawn on PL CXIX. 
illustrate the appearance presented in the case of anemophilous 
plants. Fig. 1 is the pollen-grain of the hazel, fig. 2 of the 
birch, and fig. 3 of the balsam poplar ( Populus balsamifera ), 
all nearly perfectly spherical and quite smooth, with but very 
slight angularities or protuberances. It is true the pollen of 
the hazel has been described * and even drawn as “ tri- 
angular ;” but I suspect this must have arisen from the fact of 
its having been mounted in glycerine for the microscopic slide. 
Immersion in any thick fluid has the effect of altering the 
shape of the grain, by exciting the incipient growth of the 
pollen-tubes. I have in my collection a slide of the pollen of 
Polygala myrtifolia mounted in glycerine, in which the tubes 
are as long as the longer diameter of the grain. The proper 
way of observing the form of the pollen in its condition when 
escaping from the anther — which is of course the state in which 
it is carried by the wind or by insects — is to dust it off on to a 
glass slide, and observe it at once dry and without any covering- 
glass. Examined in this way, I have found the pollen of these 
three plants scarcely to deviate perceptibly from the spherical 
* “ Nature,” vol. ix. p. 440. 
