286 TUB FLOWER. 



pollen-grains into coherent masses of various consistence, as is re- 

 markably the ease in the Orchis and Milkweed families (Fig. 543, 

 &c). Such pollen-masses are sometimes termed pollinia. 



533. The thread?, resembling cobweb, that are loosely mixed with 

 the pollen of the Evening Primrose, are the vestiges of obliterated 

 mother cells. 



534. Pollen-grains have two coats. The outer coat, called the 

 extine, is comparatively 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, or intine, which is the 

 proper cell-membrane, is a very thin and delicate, transparent and 

 colorless membrane, of considerable strength for its thickness. The 

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

 outer coat (531). 



535. The cavity enclosed by the coats is filled with a viscid liquid, 

 rich in protoplasm, which often appears slightly turbid under the 

 higher powers of ordinary microscopes, and, when submitted to a 

 magnifying power of about three hundred diameters, is found to 

 contain a multitude of minute particles (fovillce), the larger of 

 which are from the four-thousandth to the five-thousandth of an inch 

 in length, and the smaller only one fourth or one sixth of this size. 

 The smaller exhibit the constant molecular motion of all such mi- 

 nute particles when suspended in a liquid and viewed under suffi- 

 cient magnifying power. When wetted, the grains of pollen prompt- 

 ly absorb water by endosmosis (37), and are distended, changing 

 their shape somewhat, and obliterating the longitudinal folds, one or 

 more in number, which many grains exhibit in the dry state. Soon 

 the more extensible and elastic inner coat inclines to force its way 

 through the weaker parts of the outer, especially at one or more 

 thin points or pores ; sometimes forming a projection of considerable 

 length, when the absorption is slow and the exterior coating tough. 

 If the absorption continues, the distention soon overcomes the resist- 

 ance of the elastic inner coat, which bursts, and the contents are dis- 

 charged. 



536. When fresh, living pollen falls upon the stigma, however, 

 which is barely moist, it does not burst, but the inner membrane is 

 slowly projected, often through particular points, clefts, or openings 

 of the outer coat, in the form of an attenuated transparent tube (Fig. 

 537 — 547), filled with its fluid contents, and which penetrates the 

 naked and loo^e cellular tissue of the stigma, and buries itself in 



