CHAPTER 17. — SPECIAL PHYSIOLOGY OF REPRODUCTION. 769 



no doubt to the firmer cell- wall of spores, bat mainly to a differ- 

 ence in the condition of the protoplasm in the two cases : for 

 gemmae are especially adapted for rapid propagation under 

 .favourable conditions. In fact the main object attained by spore- 

 ' reproduction is the maintenance of the species through a period 

 of conditions which would be fatal to the life of the individual 

 plant. 



In Phanerogams, in which the conditions of spore-formation are peculiar 

 (see p. 431), the function of maintaining the species through a period of un- 

 favourable conditions 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 

 (p. 69) : they are developed either asexualli/ or sexually. In the 

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

 others which have become sexually degenerate (Fungi, such as 

 the ^cidiomycetes and Basidiomycetes), spores are only produced 

 asexually : whereas in sexual plants there is a sexmil^ formation of 

 \ scores, either exclusivel y (some Algae, such as the Conjugatae, the 

 • Fucaceae, and the Charoideae), or, as is more commonly the case, 

 together with asexual spore-formation. In the higher sexual 

 plants (Bryophyta, Pteridophyta, Phanerogamia) asexual spore- 

 formation is entirely restricted to the sporophyte ; whilst in the 

 lower plants in which an alternation of generation can be traced 

 (e.g. most Algae and Fungi) the gametophyte retains the capacity 

 of multiplying itself by the asexual formation of spores which are 

 distinguished as gonidia (see p. 220). The essential difference 

 between gonidia and asexual ly-produced spores is emphasised by 

 the fact that whereas the former serve merely to reproduce the 

 organism producing them, that is the gametophyte, or some form 

 of it when it is polymorphic, so that the alternation of generations 

 is unaffected, the l atter only give r i se t o the anti^thetic form in the 

 alternation of generations, that is, to the gametophyte. 



The retention by the gametophyte of the lower plants of the 

 capacity for reproduction by gonidia is of special interest in that 

 it throws light upon the course of evolution of sexual from 

 primitive asexual form s. Thus, the lowe st Algae are asexual, 

 reproducing themselves solely by asexually produced spores often 

 of the nature of zoospores. But in the course of evolution (see 

 p. 226) some of these zoospores developed into sexual reproductive 



