188 
ORGANOGRAPHY. 
BOOK I. 
the definitions required for systematic botany ; and Messrs. 
Guillemin and Adolphe Brongniart have stated that plants of 
the same family have similar pollen, adducing as instances 
Graminaceae, Cyperaceae, Thymelaceae, Proteaceae, &c., &c. 
But Mohl, who has inquired into this part of the subject in a 
most elaborate manner, declares that pollen varies extremely 
in form, not only in genera of the same family, but also in 
species of the same genus ; and that it even occurs in some 
plants that the same anther contains grains, “ de formation 
assez diverse.” The more or less complex structure of the 
pollen is not in relation to the more or less elevated station 
of a plant in the scale of developement; but the same form 
is found in families so different, that they are separated by 
every other point of structure. 
The shell of the pollen-grain appears to the observer who 
examines it with low magnifying powers, as if it were simple. 
But it has been ascertained to consist in the greater part of 
plants of two or even three membranes, of which the outer 
(extine) is thicker than the inner (intine), the latter being 
hyaline, extensible, and of extreme tenuity, not colourable by 
iodine, and destructible by concentrated sulphuric acid. Mohl 
considers the extine to be in all cases composed of minute 
grains, or cells, held together by organic mucus : that it is 
often cellular there is no doubt; he refers, in proof of the cor- 
rectness of this opinion, to cases where, as in Pitcairnia lati- 
folia, the coating is manifestly cellular in the middle of the 
pollen-grain, but becomes granular at the extremities. He 
also states, that in other cases the points forming granulations 
become less and less, till, at last, the membrane becomes 
almost entirely smooth and uniform, and is extremely like 
the membrane in the common cells of plants ; as in Allium 
fistulosum. Araucaria imbricata, &c. Mirbel, however, dis- 
putes the cellularity of the extine ; and Fritzsche, in his latest 
work, asserts that it unquestionably is sometimes a simple 
membrane (p. 30.). The intine has the power of absorbing 
water with great force, so that immediately upon being ex- 
posed to the action of a fluid it swells, and eventually bursts, 
discharii'ino- its contents; in general the extine extends as 
well as the intine, and then the organic difference between 
