PHYSIOLOGY 305 



The Union of Sexual Cells (Fertilisation). Leaving out of con- 

 sideration the necessary contributory external contrivances, fertilisa- 

 tion is accomplished by means of a chemotactic or chemotropic stimulus 

 (pp. 266, 286). It is generally the non-motile egg-cells or the female 

 sexual organ which exert an attractive influence upon the motile male 

 cells. When, however, there is no difference in the external fqrm of 

 the male and female cells, then both are usually motile, and the attrac- 

 tion seems to be exerted mutually. This is probably the case with 

 the motile and externally similar sexual cells (GAMETES) of the lower 

 Cryptogams, particularly of the Algae (Fig. 97). In the conjugation 

 of the Corijugatae, however, although both sexual cells are externally 

 alike, one cell alone is usually motile, and passes through the connect- 

 ing canal to the other. The capacity of the male cells for independent 

 movement is common to most Algae, with the exception of the 

 Florideae, in which the non-ciliated male cells are passively conveyed 

 to the female organ by the water. Throughout the whole group of 

 the higher Cryptogams, and in a few Gymnosperms, the male cells are 

 motile spermatozoids, capable of seeking out the non-motile egg-cells 

 concealed within the archegonia. But in the sexually differentiated 

 Fungi the male substance usually remains enclosed in special hyphae 

 which press themselves close against the female organs, and, by the 

 perforation of the intervening cell wall, the fusion of their contents 

 is rendered possible. The fertilisation of the Phanerogams is accom- 

 panied by a perforation of the intervening cell walls similar to that 

 which occurs in the Fungi. In this case the male cell is enclosed 

 within the microspore (pollen grain) ; the female, as a naked egg-cell, 

 is included in the megaspore (embryo-sac), which in turn lies in the 

 ovule, and in the Angiosperms the ovule is again enclosed within the 

 ovary. The double-walled pollen grains possess no independent power 

 of movement, but are conveyed to the female sexual organs by the 

 assistance of external agencies (animals, currents of air or water). 

 The pollen grain then grows out into a tube which is acted upon by 

 chemotropic (including hydrotropic and aerotropic) influences, and 

 grows like a fungal filament through the tissues of the ovary and 

 ovule until it penetrates to the egg-cell in the embryo-sac ; the 

 union of the sexual cells is then easily effected (Fig. 99). 



Recent researches have shown that in addition to the iusiou of the generative 

 cell with the ovum the second generative cell frequently fuses with the nucleus of 

 the embryo-sac. This has been termed DOUBLE-FEKTII.ISATION. The endosperm, 

 which arises from the resulting nucleus, is thus, like the embryo itself, a product of 

 fusion. This explains the "hybrid character of the endosperm in the phenomenon of 

 XI.XIA. In tin-so cases characters of the pollen-parent appear in the ripening seed 

 and not, as is usual, only in the descendants of the union. This is especially well 

 shown in the maize when, e.g. blue- and yellow-fruited races or a race the fruits of 

 which contain sugar and one in which they contain starch are interbred. It is 

 uncertain what significance is to be attached to this so-called double fertilisation ; 



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