CHAP. II. 
POLLEN. 
185 
novelty in regard to the developement of the pollen are, that, 
1. The union of pollen-grains in fours is sometimes per- 
manent, sometimes very temporary. 2. That the four are 
sometimes placed upon the same plane, sometimes in the same 
relation to each other as the four angles of a cube. 3. That 
the original number of cohering grains is in most species of 
Inga, Acacia, and Mimosa, from eight to sixteen. 4. That 
the external coat of the pollen-grain is not an uniform mem- 
brane, analogous to that of a simple cell, but an organ com- 
posed of numerous cells like the integument of an ovule, 
although it appears in some cases to be simply granular, and 
in others to be almost homogeneous. This last idea is 
sharply attacked by Mirbel (Ann. Sc.; n. s., IV. 1.), who 
insists upon the external skin of the pollen -grain being sim- 
ple in all cases. 
Mr. Griffith, in November 1836, published some curious 
observations upon this subject, the result of which is, that in 
Pardanthus chinensis the pollen is developed in the midst of 
a solid grumous semiopaque mass, forming at a very early 
stage the contents of one of the four cells of the anther ; 
That subsequently the grumous mass becomes cellular, the 
cells having undergone some separation, and consisting of a 
hyaline membrane containing a smaller mass of granular 
molecular matter. Later still, each cell, which has acquired 
an orange colour, presents traces of division into four, often 
into three, very rarely into two portions, the division being 
more distinct towards the circumference of each cell, and the 
smaller masses being each enclosed in a proper cell, but 
without having undergone any separation. Eventually each 
of the divisions becomes a pollen-grain. The young grains 
are oblong-ovate, flattened on their contiguous or inner 
faces, and open along the centre throughout the whole length 
of their outer faces. They are even at this period reticulated, 
and have rather a papillose appearance, and are lined by an 
inner membrane in the form of a hyaline sac, which bulges 
out slightly along the opening just mentioned.” 
According to Schleiden, the difference between ordinary 
pollen and that found in masses in Asclepiadaceae and Or- 
chidaceie consists in this, that the enveloping cells in common 
