CHAPTER XII 



THE SPORE-PRODUCING ORGANS 



(A) THE SORUS 



It has been seen that increase in number of individuals may be carried out 

 in the sporophyte in Ferns by separation of parts of its body {Sporophytic 

 budding, Chapter iv). The result is a mere repetition of the sporophyte. 

 Such budding is not a constantly recurring event in the life-cycle. Moreover 

 the way in which it is effected varies in different cases to such an extent 

 that its details do not form a suitable basis for a wide comparative argument. 

 It is otherwise with the alternative mode of Increase by Spores. There is 

 reason to believe that the formation of carpospores has recurred during 

 Descent, in one form or another, in each completed life-cycle since sex was 

 established. Each spore detached from the parent plant is a potential new 

 life. Not only the continuance of the species, but also its spread, are secured 

 by spores. The larger their number the more effectively are those ends 

 achieved. A recognition of this principle should underlie all comparative 

 study of the sori of Ferns. Recently it has been suggested that the larger 

 the number of tetrad-divisions in an organism where, as in Ferns, the 

 chromosomes are numerous, the larger will be the number of possible com- 

 binations of the characters which they bear, and the greater the variability. 

 Such advantages as these may be held as in some degree accounting for the 

 very large output of spores seen in homosporous Ferns. From this point of 

 view the high productivity of Ferns, their variability, and their cosmopolitan 

 distribution become intelligible. They are at once the most productive and 

 the commonest of all homosporous Pteridophytes of the present day. But 

 a primary limit to the number of their spores is imposed by the capacity of 

 the plant to nourish them. Two other considerations influence successful 

 propagation by spores. They must be properly protected till they are mature ; 

 and when ripe they must have the opportunity for free dispersal. Nutrition, 

 protection, and free dissemination are the three conditions dominating 

 successful propagation by spores. The phylesis of Ferns illustrates pro- 

 gressive changes of method by which these ends are effectively secured. 

 Those changes depend upon differences in the form, structure, and posi- 

 tion of the spore-producing organs. Such considerations as these give the 

 spore-producing organs a premier place among the distinctive characters 

 of Ferns. 



