CHAPTER XLVI. 



HETEROSPORY AND THE SEED-HABIT. 



THE theme of this book has been the origin of a Land-Flora, not the 

 examination of its ultimate developments : and accordingly the detailed 

 study has related to the homosporous Archegoniatae, with only occasional 

 allusion to those which are heterosporous, and hardly any to the Seed- 

 bearing Plants. The reason for this lies in the high degree of certainty 

 that the homosporous state was the pre-existent, and the heterosporous 

 the derivative condition from it : any study of origins will therefore relate 

 primarily to the former. But the upward evolution of Vascular Plants has 

 been intimately connected with the differentiation of the spores according 

 to sex, and the establishment of the Seed-Habit, changes which have 

 brought with them biological advantages conducing to increased precision 

 in the establishment of new individuals. The earlier step was the 

 introduction of heterospory, which results only in minor reflex effects on 

 the parent : the later adoption of the Seed-Habit has in certain cases 

 been followed by a profound modification not only of the immediate 

 spore -producing members themselves, but also of the parts which bear 

 them. It will be necessary then to, compare the condition of homosporous, 

 heterosporous, and Seed-Bearing Plants, especially with respect to questions 

 of amplification and reduction, such as have been treated of in Chapter XIX. 

 It was there concluded (p. 241) that the production of the largest number 

 of effective germs was the end of all development of the sporophyte : that 

 any increase in their number involves amplification not only of the 

 propagative system but also of the nutritive : and consequently, other 

 things being equal, there is a probability that homosporous plants as a 

 whole should illustrate lines of amplification rather than lines of reduction. 

 While admitting that reduction may occur in homosporous forms, the 

 homosporous types have for the most part been recognised as constituting 

 natural series of ascending complexity. 



The innovation of heterospory does not appear to have brought with 

 it any general reduction of parts, but changes rather of the contents 



