238 SECTIONAL ADDRESSES. 



follow in alternate succession in any normal cycle, but either may be 

 repeated indefinitely by vegetative propagation. Certain questions arise 

 with regard to the evolution of these somata as we see them. The first 

 is, how far are the diploid and haploid somata of the same cycle comparable 

 one with another ? The reply will turn upon the constancy of the events 

 of syngamy and reduction throughout descent. If they were constant, 

 then it appears a necessary consequence that the alternating diploid and 

 haploid somata must have been distinct throughout their history ; and 

 any similarity which they may show, as in Dictyota or Polysiphonia, would 

 be homoplastic. It would, indeed, appear natural that they should be 

 alike in Algee, since they are parts of the same organic life and live under 

 identical circumstances. It has, however, been suggested that reduction 

 may not be a fixed but a movable event in the individual life : liable to 

 be deferred or carried over to a later phase, in which case a diploid genera- 

 tion might arise by transformation from an already existent haploid 

 phase. The monospores of the Nemalionales have been cited as possibly 

 convertible in other Red Seaweeds into tetraspores, by some sudden 

 deferring of the act of reduction. I am not aware that this has been 

 advanced by close comparison beyond the position of tentative sugges- 

 tion, though the existence of a diploid gametophyte and of a haploid 

 sporophyte in certain abnormal ferns would indicate the possibility of 

 the suggestion being true. Pending the advance of a closely reasoned 

 argument it is best to keep an open mind." Meanwhile the weight of 

 facts hitherto known from plants at large may be held to support the 

 stability of the events of syngamy and reduction during normal descent. 

 The two generations of the same life-cycle would, in the absence of a 

 carry-over of reduction, be homoplastic, not homogenetic. 



It is, however, round a second question that divergent views as to 

 alternation chiefly centre. How far are the diploid and haploid somata 

 in the cycles of different types of organism comparable one with another ? 

 This is, in fact, the old problem of Pringsheim and Celakovsky. It applies 

 to all plants where somata alternate, but a special interest attaches to 

 the case of Land-living plants ; in particular, we shall trace the origin of 

 the dominant sporophyte of a Land Flora, and inquire whether it originated 

 by transformation of diploid developments such as are seen in certain Algae, 

 or by formation de novo through interfolation ? A clear statement of 

 the former hypothetical alternative was made by Scott in 1911, viz. 

 ' that the Fern with its stem and leaves corresponds to the Seaweed in 

 which stem and leaf are not differentiated, the whole plant being a thallus.' 

 ' On this theory the sexual prothallus and the asexual plant are both alike 

 derived from a thallus, and may once have been perfectly similar to each 

 other.' In alluding to leafy Liverworts and some of the higher Seaweeds 

 as illustrations, he then remarked that ' these are only analogies, it is 



■^ Even if such a carry-over of reduction were proved to occur in certain Algje, 

 I do not see that this would disprove interpolation in other organisms. We have 

 been too apt to assume that all alternation arose in the same way — either by inter- 

 polation or by transformation. Among the infinite possibilities of Organic Nature 

 I see little justification for assuming this. The evolutionary problem has been to 

 impose the amplification of a vegetative system upon a nuclear cycle. I see no 

 reason to exclude its solution in a multiplicity of ways. (Cf. .1. Buder, 'Zur Frage des 

 Generationswechsels im Pfianzenreiche,' Ber. a. d. Bot. Oesellach., 1916, p. 659.) 



