THE VEGETABLE CELL. 127 



vered that the embryo was not the product of the ovule, but originated in 

 the tube growing into the ovule from the pollen-grain, whence the pollen- 

 grain wab to ]>e considered as the true ovule, the plants hitherto regarded 

 as male as the female, and vice mrsd. Here agam it was Amici, who by 

 decisive observation solved the doubts arising out of this theory, and de- 

 monstrated the new doctrme to be false, a result which soon obtained full 

 confirmation from other investigations, especially from the extensive ob- 

 servations of Hofmeibter and Tulasne. 



■* The Pollen. 



As ilie development and structure of the pollen-grains have 

 already been spoken of in tlie acconnt of the development of cells, 

 I shall confine myself here to a few remarks upon this organ. 



The perfect pollen-grain consists of a cell, usually roundish or 

 elliptical (elongated into a filament in Zosteo'o), which, excepting 

 in certain water-plants, is coated on the outside by a membranous 

 layer, which owes its origin to a secretion, and, in paiiicular cases, 

 is separable into two or three superincumbent layers. The outer- 

 most membrane, corresponding to a cuticle, is mostly rather tough, 

 uniform, or covered with granules, spinules, projecting linear and 

 often reticulated ridges, mostly coloured, and the seat of a more or 

 less abundant secretion of a viscid oil. The internal coat is a co- 

 lourless, uniform, soft, and extensible cellulose membrane. Its ca- 

 vity is filled with a viscid fluid, rich in protoplasm, sometimes 

 transparent, sometimes rendered opake by granules swimming in it 

 (fovilla). In the pollen of very many plants the outer coat forms 

 one or more regularly arranged folds inwards, in which it very 

 frequently exhibits pore-like, thinner places at one or more points; 

 in hke manner, very many pollen-grains without the folds, have 

 similar pore-like" places, varying from one to a very considerable 

 number, which when large are closed by a pisce of the outer coat 

 serving as a cover. 



When a pollen-grain comes in contact with water, it powerfully 

 absorbs this, through the endosmose excited by its dense fluid con- 

 tents, swells up and tears in many places, in consequence of the 

 strong expansion its memhianes undergo through the absorption 

 of water. If the pollen-grain opposes the pressure of the absorbed 

 water through the toughness of its membrane, the inner membrane 

 is driven out, in such pollen-grain as have pore-like places in their 

 outer coat, in the form of a papilla, which often extends into a 

 rather long, cyHndrical tube (e. g,, in Dipsacese, Geraniacese, Cu- 

 cubitacese). As this phenomenon occurs in pollen-grains which have 

 been long dried, and in fact very suddenly, it can be attributed 

 only to mechanical expansion dependent on the peculiar structure 

 of the parts referred to, and not to an actual growth. 



But when firesh, living pollen comes in contact with water wMch 

 contains organic substance in solution, e. g,y with the stigmatic 

 secretion of the fluid of the nectaries of flowers, its inner coat 

 grows out, in one or more places, in the form of a tube, the length 



