1^46.] Fertilization of Plants. 53 



These pollen grains are the fine powder seen so abundantly in 

 some flowers. Under the microscope they are seen to be, usually, 

 of a spheroidal shape, and with a smooth surface; but from these 

 characters there are numerous departures, assuming in different 

 plants a great variety of form and surface. The contents of these 

 grains is called fovilla. It is a fluid holding in suspension vari- 

 ous kinds of molecules, one variety of which we need to notice. 

 They are the largest bodies seen in the fovilla, and more or less 

 cylindrical in form. They are recognized generally as the im- 

 mediate agents of fecundation. They have by some been en- 

 dowed wuth independent vitality, and have been said to exhibit, 

 under particular circumstances, vital motions. But neither of 

 these hypotheses is probably correct in the sense in which their 

 authors apply it. With regard to their vitality as independent 

 bodies, we know but little or nothing. But the motions they ex 

 hibit are the result of physical, and not of vital forces. 



The coat of the pollen grains consists of two membranes at 

 least; the outer one unyielding without being ruptured, and com- 

 paratively firm, giving form and color to the grain. The inner 

 one is very yielding, and will extend itself into tubes without 

 any lesion. The grains are exceedingly hygrometrical. When 

 placed on a moist surface, or in contact with water, they absorb 

 moisture rapidly by endosmose action, and swell very much, and 

 become nearly spherical, whatever their previous form might 

 have been. The outer membrane gives way usually at the point 

 in contact w4th the moist surface, and the inner membrane pro- 

 trudes from the aperture in the form of a tube, which is filled 

 with the contents of the grain. These are the 'pollen tubes. 



2. The necessity of the contact of the pollen grains with the 

 stigma, in order to the production of seeds, possessing vitality, is 

 abundantly proved by experiment and observation, and may be 

 most certainly inferred from the care which nature takes to bring 

 about this contact in every case in which seeds are produced. 

 The ancients seem to have had very imperfect notions with re- 

 gard to the nature of the stamens and pistil, and of the functions 

 they were destined to perform. We cannot consider the question of 



