84 BIOLOGICAL ASPECT OF ALTERNATION 



The biological, or adaptation theory of antithetic alternation in the 

 Archegoniatae, as embodied in the above paragraphs, was stated in my paper 

 on alternation published in iSgo. 1 In 1894 came Strasburger's Address 

 on Periodic Reduction, delivered at the British Association meeting at 

 Oxford. 2 He there introduced as a structural basis of antithetic alternation 

 that cytological distinction of the two generations which had already been 

 suggested by Overton : 3 this at once gave a definiteness to alternation 

 which it had never possessed before. He adopted a view similar to 

 that above stated, as a biological explanation of the rise and final dominance 

 of the sporophyte, and pointed out how its gradual development can 

 actually be traced, the first indications of it being apparently to be found 

 in the Algae : they are to be sought in such post-sexual complications, 

 connected probably with reduction, as are seen in Oedogonium. There 

 is, however, no direct evidence of the origin of any Archegoniate form 

 from any Alga : all that can be said is that, given such a multicellular 

 body as the post-sexual stage of certain green Algae, the biological conditions 

 of migration from water to land and of an amphibious life will sufficiently 

 account for the further advances which are exemplified in land-plants. 



This is, then, the working hypothesis which will form the basis for 

 our further enquiry. It will be necessary, however, to analyse the advance 

 of the sporophyte, which is thus contemplated, from its simpler beginnings, 

 and to consider the several factors which have been involved. Having 

 done this, the enquiry will be made, what evidence there is in plants, 

 living or fossil, that these factors of advance have actually been operative. 

 The initial factor appears to have been " sterilisation" that is, the delay 

 of reduction by the conversion of cells which are potentially, and were 

 ancestrally, sporogenous, into cells which serve no longer a propagative 

 but a vegetative function. It will be readily seen that this is a necessary 

 biological consequence of any considerable increase in the number of 

 spores ; and it has been pointed out above that such increase is a biological 

 advantage, especially in those plants where a land-habit places restrictions 

 upon increase in numbers by sexual propagation. The larger the number 

 of spores the greater the powers of competition, and the greater the 

 probability of survival, and of spread. On a biological theory, the nutrition 

 of the increasing number was secured by the conversion of some of the 

 potential germs to form a vegetative system, which should provide for 

 nutrition and protection. It was naturally important that these tissues 

 should be established in the individual before the sporogenous tissue 

 which it is their function to nurse : and accordingly the time of spore- 

 production was deferred, and a vegetative system, ultimately of great 



1 Annals of Botany, 1890, p. 347. 



^Annals of Botany, viii., 1894; Biol, Centralbl., Dec., 1894. A similar view has also 

 been adopted by v. Wettstein, and embodied in his Handbuch aer Syst. Bot., Band ii., 

 p. 13, published in 1903. 



3 Annals of Botany, vii. (1893), p. 139. 



