330 THE .TOUENAL OF BOTANY 



ferably generations, since the sexual phase itself is commonly 

 represented by male and female persons ; two of the generations are 

 asexual, and produce spores which express the output necessitated 

 by the dispersal function, and the amount of wastage to be counter- 

 balanced. 



The meaning of ' Alternation of Generations,' when viewed from 

 the locus of the sea and not from the standpoint of transmigrant 

 Land-Flora, is simple and illuminating. The assumption of a 

 sessile benthic state of organism as an improvement on the older 

 phase of suspended plankton, although wholly advantageous in 

 mechanism of nutrition, and leading to the specialization of the 

 efficient algal soma of marine phytobenthon, brings with it the 

 necessity of regression to the active tlagellated condition for purposes 

 of the ' sexual ' reproduction initiated and established in the ante- 

 cedent plankton-state. Hence algal phyla produce flagellated zoid- 

 gametes, some of which conjugate to give a sexually produced zygote ; 

 others remaining * apogamoiis,'' if so far failures, may nevertheless 

 ' germinate ' on attachment to a substratum to give a new individual. 



Such production of gametes fulfills two functions : — (1) that of 

 sexual fusion as the contm nation of an older plankton-phenomenon ; 

 (2) that of dispersal, a new phenomenon, first necessitated in the 

 benthic state, as a biological function of henceforward supreme sig- 

 nificance. 



As progressive differentiation of sexual mechanism leads through 

 inevitable stages of heterogamy to oogamy and fertilization in siht, 

 following the ultimate failure of the oosphere to be discharged in the 

 open medium, different phyla of algae may attain different degrees of 

 perfection in this respect. But while such fertilization in situ 

 expresses the maximum economy in reducing the wastage of the 

 sexual process, and the successful attachment of the zygote, it leaves 

 the function of dispersal wholly unprovided for. Hence the fm-ther 

 differentiation of special individuals devoted to this latter pm-pose, 

 to be taken on by the ' unilocular sporangium ' as an adapted unilocular 

 gametangium now desexed, becomes equally inevitable. All advanced 

 phyla thus tend to differentiate two generations, as the ultimate 

 response to the necessities of two great physiological functions ; one 

 secures the sexual act and chances subsequent dispersal, the other 

 secures dispersal and omits the sexual act. What was originally 

 simple differentiation {Bictyota) becomes a more exact alternation as 

 soon as fertilization in situ is established, since a sexual plant sessile 

 and parasitic on a parent would be absurd ; and all phyla of advanced 

 plant-organism present this specialization, apai't from any considera- 

 tion of what may be the state of the nuclear organization, or of 

 alternation of generations for the sake of academic considerations. 



The special point of interest of the Florideae is not so much that 

 they should have attained fertilization in situ, a parasitic zygote 

 and a * sporophyte generation ' producing asexual spores, but that they 

 now present a succession of three generations, according to the 

 scheme : — 



I. Gametophite. with sexual organs, and spermatogamic ferti- 

 lization in situ : the parasitic zygote becoming a reduced 



