774 PART IV. THE PHYSIOLOGY OF PLANTS. 



ally connected with the aquatic mode of fertilisation. It has 

 been observed and investigated in plants in which, whilst the 

 oosphere is motionless and remains in the female organ, the 

 spermatozoids are free-swimming; and it is among the most 

 striking phenomena of chemiotaxis (see p. 755). In various 

 Mosses and Ferns it has been ascertained that, on the opening of 

 the archegonium, the mncilage which is extruded includes some 

 substance which diffuses into the water and attracts to the 

 archegonium any spermatozoid that may be present ; in Mosses 

 the substance in question is cane-sugar ; in the Ferns, a salt of 

 malic acid. 



The method of bringing the spores together, so that they may 

 germinate near each other, is especially characteristic of hetero- 

 sporous plants, and more particularly of those which grow erect on 

 dry land. It is thus most strikingly exhibited by the Phanero- 

 gams, where the microspores are carried by the wind or by insects 

 into such a position that they germinate in proximity to the 

 macrospores (see p. 452). But it is not confined to land-plants, 

 for it occurs, as the result of a remarkable adaptation, in Azolla 

 (see p. 411). Nor is it exclusively restricted to heterosporous 

 plants, for cross-fertilisation in the Ferns and Mosses is largely 

 dependent upon the propinquity of a number of gametophytes 

 developed from spores shed without being scattered. 



Sexual Affinity. This term is used to express the fact that a 

 certain relationship must exist between any two gametes of oppo- 

 site sex in order that a sexual process may take place between 

 them ; when the limit is overstepped in the direction of either a 

 too close or a too remote relationship, the process will either not 

 take place at all, or the offspring will be few and feeble. 



The most fertile sexual process is that taking place between the 

 gametes of different individuals of the same species. It has been 

 proved that the offspring of such cross-fertilisation have the 

 advantage in vigour and fertility over the progeny of one of the 

 same plants when self-fertilised. It has, in fact, been proved 

 that in many Phanerogams the pollen of a flower is incapable 

 of fertilising the oospheres of its own ovules ; and that the pollen 

 from another flower of the same plant is only slightly, if at all, 

 more potent.' 



A sexual process may also take place between the gametes of 

 varieties of the same species ; of distinct species of the same 

 genus ; and even of species belonging to different genera. Such 



