268 
THE PRINCIPLES OF BOTANY. 
at once be compared with the pollen grains of the forget-me- 
not, with the -Jth inch of the same makers c, in order to 
show the enormous difference in point of size of the grains 
of fertilising powder. 
If we examine fig. a, we find it to be somewhat triangular 
in shape, consisting externally of a membrane composed of 
cellular tissue, pressed either pentagonally or hexagonally ; 
this membrane is termed extine, and is here pierced at three 
points by a more delicate expansive lining which is called 
the intine. 
Now, this arrangement is somewhat peculiar, and seems to 
be formed so as to ensure fecundation, for upon a pollen 
grain falling upon the stigma, it seems quite certain that it 
must touch between the surface at one of the angles, and 
when this occurs the point in contact elongates more and 
more, penetrating between the tissues of the style, and so 
growing downwards as a root until it arrives at the germ; 
the pollen tribe then bursts, and the contents of the pollen 
cell, fovilla acts its fecundating part. 
Now, the exact mode by which the process is brought 
about is still a matter of discussion, but it would seem that 
the formation of pollen tubes, and their action in the manner 
described, is tolerably well recognised ; and this action is 
favoured by the usually either hairy or gummy stigma—so 
made to retain the pollen grains that may impinge upon it, 
combined with a loosely-packed elongated cellular arrange¬ 
ment, in the style when that part of the organ is present. 
Now, if pollen acts in this way, it will account for the 
facility with which the gardener can form hybrids by atten¬ 
tion to fertilisation; thus, in the production of a hybrid in 
the flower to be fecundated, all its own stamens are carefully 
removed, and the pistil of that flower is fecundated by ano¬ 
ther form either of the same species or the same genus, so 
that there is a limit to the changes which can be wrought 
by this method. 
Metamorphoses or Vegetable Morphology. 
Having now described the flowers of plants, we may very 
properly enter into an inquiry upon the subject of morpho¬ 
logy ; by this we mean the reference of the forms or organs 
of plants to the leaf, and in illustrating this subject we shall 
refer to examples of morphology in the Calyx—the Corolla— 
the Stamens—and Pistils. 
Calyx .—If we stray into an old-fashioned garden, it is ten 
to one but that at this season we may find some of those 
strange polyanthus flowers in which the natural pointed 
