546 BOTANY OF THE LIVING PLANT 



which archegoniate organisms are exposed. The problem before the nascent 

 plant possessing sexuality has been to adjust its somatic development to the 

 nuclear cycle. But these are two essentially different things. The somatic 

 phases must undergo development in relation to external conditions, whereas 

 the normal succession of syngamy and meiosis in the nuclear cycle is not 

 affected by them. There is thus no ground for assuming that the two should 

 be interlocked in any rigid scheme, such as that presented by the archegoniate 

 cycle. All life-cycles do not, as a fact, lend themselves to interpretation in 

 terms of the Hofmeisterian Cycle. While " antithetic alternation " is exem- 

 plified in a high degree by the vegetation of the land, it should not be forgotten 

 that it is also characteristic of some of the Brown Algae, e.g. the Laminariaceae. 

 On the other hand, " homologous alternation " is seen only among the Thallo- 

 phyta, and particularly the Algae. Such facts uphold the conclusion of Olt- 

 manns, that just as sexuality may be held to have arisen repeatedly and 

 independently in various groups of the lower organisms, so may various higher 

 families have carried out independently the establishment of two generations. 

 Continuing this line of thought Von Goebel has affirmed that the doctrine of 

 alternation founded originally for the Higher Plants (from the Bryophyta 

 upwards) cannot be extended to all plants. From such views it follows that 

 the relatively stable alternation, as described by Hofmeister for the Arche- 

 goniatae, should be discussed separately from others, and on its own merits. 

 To secure a clear verbal contrast it will be well to drop the old terminology, 

 which never was explicit ; and to substitute " interpolation theory " in place 

 of antithetic, and " transformation theory " for homologous. These words 

 are explicit in conveying the opposed theories of origin of the alternation, 

 which differ in the fixity or otherwise of somatic development in relation to 

 syngamy and meiosis. 



Among the Algae somatic alternation may be absent. In those Algae 

 which have been distinguished as haplobiontic, the life-cycle, as in the Red 

 Alga Scinaia and a majority of Green Algae, includes only a single somatic 

 phase ; and this is usually haploid. On the other hand the life-histories of 

 Dictyota and Polysiphonia are diplobiontic, with regularly alternating haploid 

 and diploid somata, though these are uniform in outline and evenly balanced. 



With the ground thus freed from the trammels of the old assumption of 

 uniformity in alternation, we may next enquire into the subaerial conditions 

 under which the diplobiontic type of alternation seen in the Archegoniatae 

 originated. It appears that the effect of Life on Land has been to stabilise the 

 somatic and cytolcgical relations of the Hofmeisterian Cycle. We may picture 

 to ourselves the working out of the conditions affecting their early evolution as 

 follows. Those simple Archegoniatae which formed the primitive Flora 

 of the Land probably sprang from green haplobiontic Algae, inhabiting 

 shallow fresh water or the higher levels between tide-marks. Here sexual 

 reproduction would be effected through the medium of external liquid water. 

 If other conditions were favourable this could be carried out by them at any 

 time, provided water be present and the sexual organs mature. But certain 

 types escaped competition or availed themselves of a varying water-level, 

 by establishing themselves on land where access to liquid water was only 

 an occasional occurrence. In these the sexual process would only be effected 

 at times of rain or copious dews. Less dependence could then be placed upon 



