258 NATURE AND FORMATION OF THE POLLEN. 
at the circumference, as in Liuhea paniculata (fig. 557); or the 
reverse may be the case, as in many of the Rosaceze. In the 
case of diplostemenous flowers, as with the Willow Herb (Epilo- 
biwm), the stamens alternating with the petals are almost always 
longer than those opposite to them. When the stamens are of 
unequal length in the same flower, or in different flowers of the 
same species, as in the Primrose, they are said to be dimorphic, 
and will be afterwards alluded to in speaking of fertilisation. 
When there is a definite relation existing between the long 
and short stamens with respect to number, certain names are 
applied to indicate such forms of regularity. Thus in the Wall- 
flower (figs. 26 and 558), and Cruciferous plants generally, there 
are six stamens to the flower, of which four are long and ar- 
ranged in pairs opposite to each other, and alternating with two 
solitary shorter ones ; to such an arrangement we apply the term 
tetradynamous. When there are but four stamens, of which 
two are long and two short, as in Labiate plants generally (jigs. 
485 and 487), and in the Foxglove (jig. 559), and most other 
Scrophulariaceous plants, they are said to be didynamous. 
THE Potten.—The pollen consists of microscopic cells, which 
correspond to the microspores of the higher Cryptogams. It has 
also been stated, that the pollen was formed in certain cells de- 
veloped originally in the centre of the parenchyma of the lobes 
of the young anther (fig. 520, em); also that these cells were 
enclosed in a special covering of their own, cl, and that in the 
course of growth they pressed upon the coats of the anther, so 
as to cause their more or less complete absorption, and finally 
assisted in promoting the dehiscence of the anther (page 251). 
We have now more particularly to describe the mode of forma- 
tion and the structure of the pollen. 
Formation of the Pollen.—The formation of the pollen may be 
described as follows :—The large cells (fig. 520, em), which are 
developed in the parenchyma of the young anther, and which 
are destined for its formation, are called parent or mother cells ; 
these are surrounded in the earlier stages of development by a 
single stratum of thin-walled cells forming an internal epithelial 
layer or tapetuwm, cl, which, however, becomes subsequently 
pressed upon and absorbed. Usually these parent cells remain 
connected to one another, but in some instances, and more espe- 
cially in Monocotyledons, they become isolated and float free in 
a more or less viscid material occupying the cavity of the anther. 
As development proceeds the nucleus of each parent cell dis- 
appears, and in its place four new nuclei are ultimately formed 
(fig. 560, a). (See ‘ Karyokinesis ’in Physiology.) Then follows 
an infolding of the protoplasm, or, according to Mohl, of the 
primordial utricle, a, b, c, by which the mother-cell is either 
divided at once into four parts; or first into two, and subse- 
quently, each of these again divided into two others. The four 
cells thus formed become each surrounded by a cellulose mem- 
