98 POLLINATION AND FERTILIZATION 



Fixation of the Pollen. — The pollen thus transferred to the stigma 

 must be fixed there in order that fertilization may follow pollination. 

 This process is effected by contrivances little less elaborate, although 

 more minute, than those which have been described. These con- 

 trivances relate in part to peculiarities of the pollen. As regards the 

 stigma, fixation is effected most generally by means of the viscid 

 secretion to which reference has been made, the stigma being essentially 

 glandular in nature. Appendages in the form of hairs, scales, or 

 papillae (Fig. 27.5) are very common. In some cases the divisions of 

 the stigma are sensitive and close elastically upon the pollen as soon 

 as it is deposited. With the fixation of the pollen upon the stigma, 

 polhnation is completed and preparations for fertilization begin. 



FERTILIZATION 



A knowledge of fertilization is of importance to the pharmacognosist 

 only as it throws light upon the characters of the fruit, in which we 

 include the seed as a part. Only the principal facts connected with the 

 subject will, therefore, be here considered. 



Internal Structure of the Ovule. — The gross appearance and parts of 

 the ovule have already been described. Its internal structure is illus- 

 trated in Fig. 277. 



The immediate function of the flower has been seen to be the pro- 

 duction of spores. These spores are to act as reproductive bodies, which, 

 like seeds, they can do only by germinating and growing in a suitable 

 soil. This function of each will now be considered. 



We have seen that the macrosporophyll is the carpel, its macro- 

 sporange the ovary. The macrospore itself is tlie large centrally 

 located cell of the nucellus, which is to develop into the embryo-sac, e. 

 The natural soil for the germination of this spore is the tissue of the 

 nucellus where it is formed. Its germination takes place immediately 

 and results in the development of the several distinct bodies figured in 

 the illustration. Of these bodies, the oospore or vegetable egg, o, is 

 the ultimate female reproductive element. 



The Gymnospermous Ovule. — The ovule of gymnosperms agrees in 

 the possession of an embryo-sac, with several bodies corresponding to 

 the oosphere of angiosperms, but with the other corpuscles not clearly 

 developed. The foramen is secretory, so as to be adapted to acting 

 upon the pollen-grain which it receives, as does the stigma in angio- 

 sperms. 



