230 PART III. PHYSIOLOGY. [ 56 



56. SpOKe-Reproduction (see p. 50). The highest degree 

 of reproductive capacity is that possessed by spores. Though 

 they are single cells, they are nevertheless capable, each by it- 

 self, of giving rise to a plant-body which, as in the higher plants, 

 may present complete morphological and histological differentiation. 



The advantages gained by the development of spores are, first, 

 that they are readily scattered, so that the plants developed from 

 them grow at a distance from each other ; this is, for instance, 

 the meaning of the development of free swimming zoospores by 

 plants (Algae) living in water. Secondly, spores, especially in the 

 lower plants, are highly resistent to unfavourable conditions, such 

 as drought and extremes of temperature ; so that they serve to 

 maintain the species under conditions which would be fatal to the 

 plant itself. 



In Phanerogams the function of maintaining the species through a 

 period of unfavourable conditions, as also the dissemination of the new 

 plants, is transferred to the seeds which, like the spores of lower plants, 

 have a great capacity for endurance. 



Most plants, and probably all, produce spores ; and from the 

 physiological point of view there are two modes of origin of 

 spores : they are developed either asexually or sexually. In the 

 lowest plants (e.g. Cyanophyceae, Schizomycetes, etc.), as also in 

 others which have become sexually degenerate (Fungi, such as 

 the J^cidiomycetes and Basidiomycetes), spores are only produced 

 asexually : whereas in some sexual plants there is an exclusively 

 sexual formation of spores (some Algae, such as the Conjugates, the 

 Fucacese, and the Charoideae). In the higher plants (Bryophyta, 

 Pteridophyta, Phanerogamia) spores are produced both sexually 

 and asexually. 



Sexual Spore-formation. The sexual process consists typi- 

 cally in the fusion of two gametes, that is, of two sexual reproduc- 

 tive cells, neither of which is capable, by itself, of developing into 

 a new individual. 



The first question which naturally arises is as to the nature of 

 sexuality ; the question, namely, as to what difference, if any, can 

 be observed between a gamete and an asexually-produced spore. 

 To this question no answer can at present be given ; no difference 

 can be detected between a gamete and a spore. It must not, 

 however, be concluded that because there is no observable differ- 

 ence between a gamete and an asexually-produced spore, there is 

 no difference whatever between them; on the contrary it is clear 



