PHYSIOLOGY 281 



As it is thus necessary in sexual reproduction not only to provide 

 for the production of male and female cells, but also to ensure their union, 

 it becomes at once evident that, for sexual reproduction, the organs 

 must have a different morphological and anatomical structure than if 

 they were designed solely for vegetative activity. The sexual organs 

 accordingly often exhibit a special and peculiar form, which differs 

 materially in appearance from the vegetative parts of a plant. 



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

 sideration the necessary external contrivances to that end, fertilisation 

 is accomplished by means of a chemotactic or chemotropic stimulus 

 (pp. 243, 263). It is generally the non-motile egg-cells or female 

 sexual organ which exert an attractive influence upon the motile 

 male cells ; as, for instance, in the case of the Mosses, where the sper- 

 matozoids are enticed within the archegonia by a solution of cane- 

 sugar, or, as in Ferns, where they are similarly stimulated by malic 

 acid. When, however, there is no difference in the external form 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. 69). In the conjugation of 

 the Oonjugatae, however, although both sexual cells are externally alike, 

 one cell alone is usually motile, and passes through the connecting 

 canal to the other ; and in the Fucaceae, though the egg-cells are 

 ejected from the mother plant, they have not themselves any power 

 of movement, while the male cells or spermatozoids, by means of their 

 cilia, are capable of independent motion. This capacity of the male 

 cells for independent movement is common to most Algae, with the 

 exception of the Florideae, by which the walled male cells are passively 

 conveyed to the female organ by the water. Throughout the whole 

 group of the higher Cryptogams, the male cells are motile spermato- 

 zoids, 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 hyphse which press them- 

 selves 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 accompanied 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 pollen grain ; the female, 

 as a naked egg-cell, is included in the 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 fungus-filament through the tissues of the ovary and 



