232 PART III. PHYSIOLOGY. [ 56 



striking phenomena of chemiotaxis (see p. 220). In various 

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

 the archegoninm, the mucilage 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 in the pollination 

 of the Phanerogams, where the microspores are carried by the wind 

 or by insects into such a position that they germinate in proximity 

 to the macrospores. 



In order that a sexual process may take place between them, a 

 certain relationship must exist between any two gametes of oppo- 

 site sex ; 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 

 Bame 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 varieties of the 

 same species ; between distinct species of the same genus ; and 

 even between species belonging to different genera. Such a process 

 is known as hybridisation, and the progeny as hybrids, the hybrid 

 being distinguished as a variety- hybrid, species-hybrid, or genus- 

 hybrid, according to circumstances. 



Effects of the Sexual Process. The sexual process is not always 

 limited in its effect to the production of a spore which will give 

 rise to a new individual. For instance, when the female cell is 

 borne by the parent at the time of fertilisation, the act of fertili- 

 sation induces a more or less marked growth and change in the 

 adjacent organs and tissues of the parent, leading to the formation 

 of & fruit (see p. 61). 



