THE POLLEN -TV BE 99 



The Female Gametophyte. — As is the product of seed-germination, 

 so is the product of spore-germination a plant. The plant which results 

 from seed-germination, and which produces spores, has been called a 

 sporophyte. That which results from spore-germination, and the 

 ultimate function of which is the production of seed, is called a gameto- 

 phyte. Hence the mass contained within the embryo-sac is such a 

 plant, the female gametophyte. It will rest in the state in which we 

 now find it until its oosphere is acted upon by the male element, which 

 we have yet to consider, and if such action does not occur, it will die 

 and disappear. 



Connection between Stigma and Ovule. — Between the ovule thus 

 prepared and the stigma, there is an almost continuous connection 

 through condu'cting tissue, extending through the body of the stigma, 

 style, and placenta. The extent of this conducting tissue, like that of 

 the stigmatic surface, is usually greater or less according to whether 

 there are more or fewer ovules to be fertilized. 



We have seen that the soil upon which the microspore is intended to 

 germinate is the stigma, in angiosperms, and the foramen of the ovule 

 in gymnosperms. The process of germination is dependent upon the 

 following structural characters: 



Structure of the Microspore. — The pollen-grain consists of a highly 

 hygroscopic mass of tissue, partly vital and partly nutritive, the latter 

 of variable composition, surrounded by a thin, non-perforated, highly 

 elastic membrane, the Intine, and this in turn by a thicker, non-elastic 

 covering, the Extine, or "Exine," bearing one or more complete per- 

 forations, very thin places, or otherwise modified points upon its surface. 

 In exceptional cases there is instead but a single wall. 



Germination of the Microspore. — The process of fertilization is illus- 

 trated by Fig. 278, and the ordinary phenomena are as follows: The 

 pollen grain (a), fixed upon the stigma of the angiosperm, or upon the 

 summit of the ovule of the gymnosperm, the hygroscopic contents 

 absorb moisture from the secreting or transuding surface with which 

 it is in contact, the mass increases in size and distends the intine which 

 surrounds it. Shortly cell-division of its contents takes place, the com- 

 bined changes constituting the germination of the microspore. 



The Pollen-tube. — Through one or more of the perforations of the 

 extine already existing, or forcibly made by this process, protrude 

 prolongations of the pollen contents, still enveloped in a process of the 

 intine. Such a prolongation is known as a Pollen-tube (6). 



