POLLEN. 



257 



Cichory and Thistle tribes, many-sided ; in the Musk-plant, 

 spirally grooved ; in the Mallow family and the Squash and 



517 



521 



Pumpkin, beset with briskly projections, &c. The pollen of 



Pine, as well as that of the Onagracese, is not so simple, but 



appears to consist of three or four blended cells ; 



that of most Ericaceae evidently consists of four 



grains or cells united. (Fig. 512-521.) The 



most extraordinary shape is that of Zostera, or 



the Eel-grass of salt-water, in which the grains 



(destitute of the outer coat) consist of long and 



slender threads, which, as they lie side by side in the anther, 



resemble a skein of silk. 



471. Pollen-grains are usually formed in fours, by the division 

 of the living contents of mother cells fu\st into two, and these 

 again into two parts, which become specialized cells. As the 

 pollen completes its growth, the walls of the mother cells are 

 usually obliterated. But sometimes these cells 



persist, either as shreds, forming the cobweb-like 



threads mixed with the pollen of Evening Primrose, 



or as a kind of tissue combining the pollen into 



coherent masses, of various consistence. Of this 



kind are the elastically coherent pollen-masses (or 



POLLINIA, sing. POLLINIUM) of Orchises (Fig. 463), 



and the denser waxy ones of mairy other orchids and those of 



Asclepias or Milkweed, Fig. 522. 



472. A pollen-grain has two coats. The outer coat is com- 

 paratively thick, and often granular or fleshy. This is later 

 formed than the inner, and by a kind of secretion from it : to it 

 all the markings belong. The inner coat, which is the proper 

 cell-wall, is a very thin, delicate, transparent and colorless mem- 

 brane, of considerable strength for its thickness. The pollen 

 of Zostera and of some other aquatic plants is destitute of the 

 outer coat. 



473. The cavity enclosed by the coats is filled with a viscid 

 substance, which often appears slightly turbid under the higher 

 powers of ordinary microscopes, and, when submitted to a mag- 



FIG. 516-521. Forms of pollen: 516, Lily; 517, Cichory; 518, Pine; 519, Circrea; 

 520, Kalmia; 521. Evening Primrose. 



FIG. 522. A pair of pollinia of Asclepias, annexed by their caudicles to the gland. 



17 



