258 THE FLOWER. 



nifying power of about three hundred diameters, is found to 

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

 which are from one four-thousandth to one five-thousandth of an 

 inch in length, and the smaller only one fourth or one sixth of 

 this size. When wetted, the grains of pollen promptly imbibe 

 water by endosmosis, 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 projections, 

 when the absorption is slow and the exterior coating tough. In 

 many kinds of pollen, the grains, when immersed in water, soon 

 distend to bursting, discharging the contents. 1 



474. Pollen- tubes. In others, and in most fresh pollen, when 

 placed in ordinarily aerated water, at least when this is slightly 

 thickened by sj'rup or the like, and submitted to a congenial tenv 

 perature, a projection of the inner coat through the outer appears 

 at some one point, and by a kind of germination grows into a 

 slender tube, which may even attain two or three hundred times 

 the diameter of the grain ; and the richer protoplasmic contents 

 tend to accumulate at the farther and somewhat enlarging ex- 

 tremity of this pollen-tube. 2 



475. In cleistogamous flowers (434), the pollen, while still in 

 the anther, sends out its tubes, which may grow to a great length, 

 in the mere moisture of the flower-bud, the growing tip always 

 directing itself toward the stigma in a wonderful way. Similarly, 

 in the open flower of Milkweeds, the pollen-tubes sometimes 

 start from the pollen-mass even while yet in the anther, and in 

 vast numbers, forming a tuft or skein of pollen-tubes, which 

 may attain considerable length and direct itself toward the some- 

 what distant stigma. Commonly, however, the pollen remains 



1 In Coniferae, the grams of pollen have a peculiar internal structure or 

 rather a development (suggestive of a homology with the microspores of some 

 of the higher Cryptogamia), the contents at or before maturity undergoing 

 division into two or three internal cells, only one of which acts in fertiliza- 

 tion. When they act upon the ovule or are placed in water, and the inner 

 coat swells by absorption, the bursting outer coat is commonly thrown off. 

 In Pines and Firs (but not in Larch and Hemlock Spruce), the grain of 

 pollen is singularly compound, consisting (as in Fig. 518) of a central arcuate 

 body (the proper pollen-cell) bearing at each end an empty roundish cell. 

 These are vesicular protrusions or appendages of the proper pollen-grain, of 

 no known functional importance, except that they render such wind-dis- 

 persed pollen more buoyant for transportation. 



a Van Tieghem, in Ann. Sci. Nat. ser. 6, xii. 312, &c., 1869. 



