XIII] IMPORTANCE OF LARGE SPORE-OUTPUT 271 



capacity. The most effective feature introduced in its later stages lies in 

 the spread of the physiological drain of spore-production over a lengthened 

 period of time, by the introduction of the gradate and mixed conditions. 

 The interdependence of soral and sporangial progress thus led to this final 

 result ; that of all the homosporus Pteridophytes, the Filicales, whether 

 Eusporangiate or Leptosporangiate, include the most successful exponents 

 of the method of survival and spread of the race that depends primarily on 

 spore-number. The high multiplication of individual chances of success in 

 the Ferns may be held as an offset against the risks of a very vulnerable life- 

 history. Their prevalence at the present day shows that the balance has 

 been in their favour. 



It may be that increase in number and in spread of individuals may 

 not have been the sole, or even the initial reason for the high output of 

 spores seen in Ferns. It has been suggested by Svedelius {Ber. d. D. Bot. 

 Ges. 192 1, Bd. xxix, p. 178) that the most important feature of the reduction- 

 division lies in the possibility of producing new combinations of chromosomes 

 in the daughter-nuclei : and that the more numerous the spore-mother-cells 

 the more frequently acts of reduction will be effected, and the possible 

 number of combinations increased. In this he sees a possible explanation 

 of the origin of the diploid " soma," or sporophyte : for thereby numerous 

 reduction-divisions are secured. The result should be, especially where the 

 number of chromosomes is large, a high degree of variability in the progeny, 

 which is certainly the case in Ferns. Though this theory may offer a new 

 aspect of the origin of a somatic sporophyte, increased variability cannot be 

 regarded as the sole factor which has led to its extension, and finally to its 

 high spore-output, as in Ferns. It seems impossible to ignore for them the 

 effect of their high fecundity as in itself a positive advantage, specially 

 fitting them for survival under competition, and for spread to new and 

 untenanted areas. 



BIBLIOGRAPHY FOR CHAPTER XIII 



211. Bower. Studies in the morphology of Spore-Producing Members. II. Dulau. 

 London, 1896. Ill and IV. Phil. Trans. Vol. 189 (1897), Vol. 192 (1899). 



212. Bower. Studies in the Phylogeny of the Filicales. I — VII. Ann. of Bot. 1910 — 

 1918. 



213. Bower. Origin of a Land Flora. 1908, pp. 637 — 646. 



214. Engler & Prantl. Natiirl. Pflanzenfam. i, 4, p. 79, etc. Here the literature up 

 to 1902 is fully quoted. 



215. Kny. Botanische Wandtafeln. Text, p. 418, etc. 1895. 



216. Campbell. Mosses and Ferns. 2nd Edn. (1905), where full references to the 

 literature are given. 



217. Von Goebel. Organographie. 2nd Edn. (1918), p. 1159, etc. 



