Nos. 618-619] THE ROLE OF REPRODUCTION 



276 



carrying on their vegetative functions. These come to- 

 gether in pairs. Two individuals become one as a pre- 

 requisite to renewed vigor. Vegetative spores become 

 gametes. Something valuable— speed of multiplication 

 —is given up for a time that something more valuable in 

 the general scheme of evolution may be attained. 



This is indeed an alluring genesis of sex. Let us use 

 the indefinite article, however; no doubt it is a genesis of 

 sex, but it can hardly be the genesis of sex. Various mani- 

 festations of sex are present in other widely separated 

 groups of unicellular plants, the Peridinea\ the fon.jngata' 

 and the Diotomeae— the Conjugata> being indeed the onlv 

 great group of plants in which there is no asexual repro- 

 duction. In these forms one can not make out such a good 

 case of actual gametic origin, but the circumstantial evi- 

 dence of sex development in parallel lines is witness of its 

 paramount importance. 



After the origin of sex. many changes in reproductive 

 mechanisms occurred in plants, but almost all of them 

 resulted merely in greater protection of the gametes, in 

 increased assurance of fertilization, or in provision for 

 better distribution. First there was a visible morpho 

 logical differentiation of gametes, the one becoming a 

 large inactive cell stored with food, the other becoming 

 small and mobile. Then came the evolution of various 

 organs, and finally the alternation of generations. In the 

 higher plants a long line of changes have occurred con- 

 nected with the alternation of generations ; the spore-pro- 

 ducing type has developed from a form of little impor- 

 tance to that which dominates the vegetable world, the 

 gamete-producing type has degenerated until it consists 

 of but two or three cell divisions. In these variation- 

 there is reproductive insurance, something which also may 

 be said of those manifold adaptations which provide 

 zygotic protection either in the seed or the adult plant, 

 but they are no more direct changes in reproductive 

 mechanism than are the diverse means which arose to 

 secure dispersal. In fact in all of these changes no new 



