THE LICHEN LIFE-CYCLE 169 



economy of which the vegetation of the sea is capable. Economy of 

 the wastage of gametes thus passes far beyond the mere heterogamy 

 of the Phseophyeeae, so admirably illustrated by the highest Fucoids 

 of the Brown alliance (Saryassum, Himanthalia), to fertilization 

 in situ, and all following stages of post-sexual nutrition of the 

 zygote— to a condition, in fact, almost comparable with that of the 

 Angiospermous Flowering-plant, in which the gametes are now 

 reduced to the state of mere undifferentiated nuclei. The latter, 

 again, are produced in the Floridese within reproductive organs also 

 with a minimum of differentiation. In fact, the procarpial mechanism 

 of the Floridese is readily identified in practice, not by looking for 

 the actual carpogonium, but for the characteristic vestigial ramulus of 

 an older uniseriate filamentous construction which remains as gameto- 

 phore. 



The point of departure for all this progression is evidently to be 

 found in the institution of spermatogamy, as replacing the zoidogamy 

 of older flagellated races, still retained dominant in the Phseophyeeae. 

 The immature antheridial ramulus, or its uninucleate contents, is 

 precociously discharged in the manner of the emission of the antheri- 

 dium of Fucus, and the mucilaginous distal end of the oo^onial hair- 

 termination becomes a receptive ' trichogyne '-apex with which con- 

 jugation may be readily effected. By such fertilization in situ all 

 wastage of fertilized oospheres is eliminated, and cheap minimum 

 units conveying the male gamete-nucleus are produced in a quantity 

 sufficient in a gently moving medium, even without the differentiation 

 of motor-flagella, to be fully effective in promoting fertilization 

 provided a satisfactory plankton-rate be attained. This last factor 

 affords the clue to the situation. Flagellar activity, the inheritance 

 of a bygone plankton-phase and its photosynthetic problems and 

 apparently never effective to a rate of more than a foot per 'hour, 

 is clearly of little value as the main mechanism of sexual approxi- 

 mation in rough surface-waters on a rocky shore. The flao- e lla of 

 the zoiids of such a reef-pool form as Dictyota are already"clearly 

 deteriorated. Complete loss of all such flagellation will be equally 

 disadvantageous in dead still water ; though the latter condition i's 

 probably never attained in the sea, and even in the quietest sunlit 

 pond there will be convection-currents in operation sufficient to move 

 minute reproductive units of density practicallv similar to that of the 

 medium. The quiet pool becomes the limit of aqueous environment 

 whether in tide-pool formation, or on tropical coral-reefs where an 

 oceanic tide may be negligible. This is the habitat which has pro- 

 duced the very precise type of fertilization in the Florideae, however 

 much stray members of the group may survive in subsidiary stations 

 even in Arctic seas (PJiycodrys), or be relegated to deep quiet levels • 

 and it begins to be clear that all other spermatogamic races must 

 have passed through the same environment, wherever they may be 

 found established in the present world. Similar phenomena of sperm- 

 atogamy thus tracing their inception to the pool-formations of the 

 sea-margin, in which a sufficient plankton -rate may be attained 

 within the possibilities of the dilution of a comparatively small 

 volume of water at little expense,— the average plankton-rate of a 

 Journal of Botany. — Vol. 59. [June, 1921] n 



