358 SHELL OF POLLEN-GRAIN. [BOOK i. 



the same family have similar pollen, adducing as instances 

 Grasses (Graminacese), Sedges (Cyperacese), Daphnads (Thy- 

 melacese), Proteads, &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 

 development; 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 correctness of this opinion, to cases where, as in Pit- 

 cairnia latifolia, 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, disputes the cellularity of the extine; and Fritz- 

 sche, 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 imme- 

 diately upon being exposed to the action of a fluid it swells, 

 and eventually bursts, discharging its contents ; in general 

 the extine extends as well as the intine, and then the organic 

 difference between them is not observable : but in the Yew 



