FERTILIZATION. 



43 



FERTILIZATION. 



A knowledge of fertilization is of im- 

 portance to the pharmacognosist only as 

 it throws light upon the characters of the 

 frultj in which term we include the seed. 

 Only the principal facts connected with 

 the subject will, therefore, be here consid- 

 ered. 



The gross appearance and parts of the 

 ovule have already been described. Its 

 internal structure is illustrated in Fig. 

 238. 



g 



:z39. 



The Nucellus, being a cellular body, in 

 which one or more spores are to develop, is 

 a Sporangium. The development of its 

 solitary spore presents striking differ- 

 ences from that in flowerless plants, the 

 most important result being that the spore 

 produced is single, and, germinating, fills 

 the entire interior of the nucellus, the 

 cellular tissue surrounding it at the time 

 of its origin mostly disappearing subse- 

 quently to give place to it. Almost imme- 

 diately after its formation the macrospore 

 germinates by the division of its nucleus. 

 Although the development is very simple 

 and the growth very slight during the first 



stage, the resulting structure confined en- 

 tirely within the embryo-sac, it is to be 

 regarded as a plant body, the female 

 gametophyte. It develops seven incom- 

 pletely formed cells, or corpuscles, pos- 

 sessing specialized functions. That occu- 

 pying a central position is the Central Cell 

 or Nucleus of the Embryo-Sac. The two 

 nearest and in contact with the foramen 

 are the Synergidae, in contact with which 

 lies one called the Egg-Cell, or Oosphere, 

 Those at the opposite end of the embryo- 

 sac are called the Antipodal Cells. -*ie 

 month of the foramen affords a means of 

 ingress to the fertilizing element in inis, 

 the angiospermous ovule. The ovule of 

 gymnosperms agrees in the possession of 

 an embryo-sac, with several bodies corre- 

 sponding to the oosphere of angiosperms, 

 but the other corpuscles not clearly devel- 

 oped. The foramen is secretory, so as to 

 be adapted to acting upon the pollen-grain 

 which it receives, as does the stigma in 

 angiosperms. 



Between the ovule thus prepared and 

 the stigma there is an almost continuous 

 connection through conducting tissue, ex- 

 tending through the body of the stigoia, 

 style and placenta. The extent of this 

 conducting tissue, like that of the ftig- 

 matic surface, is usually greater or less 

 according to whether there are more or 

 fewer ovules to be impregnated. 



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, according 

 to some writers, "Exine," bearing one of 

 more complete perforations, very thin 

 places or otherwise modified points upon 

 its surface. In exceptional cases the pol- 

 len grain possesses but a single wall. 



The process of fertilization is illustra*:ed 

 by Fig. 239, and the ordinary phenomena 

 are as follows:— The pollen grain, fixed 

 upon the stigma of the angiosperm, or 

 upon the summit of the ovule of the 

 gymnosperm, the hygroscopic contents 

 absorb moisture from the secretiug or 



with which it is in 



transuding surface 



oxidation and 



nutrition 



contact, oxidation ana nutrition com- 

 mence, the mass increases in size, iind 

 distends the intine which surrounds it. At 



