October i, 1896J 



NA TURE 



537 



another at the base of the series, for the regular alternation of 

 the Bryophyta is missing in the Alg?e and Fungi, and the 

 question as to what corresponds among these lower groups to 

 the sporophyte and oophyle of the higher Cryptogams is still 

 disputed. 



Now as regards this life-cycle, which is characteristic of all 

 plants higher than Algoe and Kungi, there are two great questions 

 at present open. The one is general : are the two generations, 

 the sporophyte and the oiiphyte, homologous with one another, 

 or is the sporophyte a new formation intercalated in the life- 

 history, and not comparable to the sexual plant ? The former 

 kind of alternation has been called homologous, the latter 

 antithetic. This question involves the origin of alternation ; its 

 solution would help us to bridge over the gap between the 

 Archegoniat.v and the lower plants. The second problem is 

 more special : has the sporophyte of the Pteridophyta, which 

 always appears as a complete plant, been derived from the 

 simple and totally different sporophyte of the Bryophyta, or are 

 the two of distinct origin ? 



At present it is usual, at any rate in England, to assume the 

 antithetic theory of alternation. Prof. Bower, its chief exponent, 

 says (" Spore-producing Members," Phil. Trans., vol. clxxxv. 

 B. (1894), p. 473) : " It will also be assumed that, whatever may 

 have been the circumstances which led to it, antithetic alternation 

 was brought about by elaboration of the zygote {i.e. the fertilised 

 ovum] so as to form a new generation (the sporophyte) inter- 

 polated between successive gametophytes, and that the neutral 

 generation is not in any sense the result of modification or 

 metamorphosis of the sexual, but a new product having a distinct 

 phylogenetic history of its own." In his essay on " Antithetic 

 as distinguished from Homologous Alternation of Generations in 

 Plants'' ("Annals of Botany," vol. iv. (1890) p. 362), the 

 author describes the hypothetical first appearance of the 

 sporophyte as follows : " Once fertilised, a zygote might in these 

 plants [the first laiid plants] divide up into a number of portions 

 (carpospores), each of which would then serve as a starting- 

 point of a new individual." 



On this view, the sporophyte first appeared as a mere group 

 of spores formed by the division ot the fertilised ovum. 

 Consequently the inference is drawn that all the vegetative parts 

 of the sporophyte have arisen by the " sterilisation of potentially 

 sporogenous tissue." That is to say, there was nothing but a 

 mass of spores to start with, so whatever other tissues and 

 organs the sporophyte may form must be derived from the 

 conversion of spore-forming cells into vegetative cells. Prof. 

 Bower has worked out this view most thoroughly, and as the 

 result he is not only giving us the most complete account of the 

 development of sporangia which we have ever had, but he has 

 also done much to clear up our ideas, and to show us what the 

 course of evolution ought to have been if the assumptions 

 required by the antithetic theory were justified. 



Without entering into any detailed criticism of this important 

 contribution to morphology, which is still in progress, I wish to 

 point out that we are not, after all, bound to accept the assumption 

 <m which the theory rests. There is another view in the field, 

 for which, in my opinion, much is to be said. The antithetic 

 theory is receiving a most severe test at the friendly hands of its 

 chief advocate. Should it break down under the strain we need 

 not iles|iair, for another hypothesis remains which I think quite 

 equally worthy of verification. 



This is the theory of I'ringsheim, according to which the two 

 generations are homologous one with another, the oiiphyte 

 corresponding to a sexual individual among Thallophytes, the 

 -sporophyte to an asexual individual. To quote Pringsheim's 

 own words ("Gesammelte .-\bhandlungen," II. p. 370) : ''The 

 alternation nf generations in mosses is immediately related to 

 those phenomena of the succession of free generations in 

 Thallophytes, of which the one represents the neutral, the other 

 the sexual plant." Further on (ibid., p. 371) he illustrates this 

 by saying : " The moss sporogonium stands in about the same 

 relation to the moss plant as the sporangium-bearing specimens 

 of Saprokgnia stand to those which bear odgonia, or as, among 

 the Floridea;, the specimens with tetraspores are related to those 

 with cystocarps." This gets rid of the intercalation of a new 

 generation altogether ; we only require the modification of the 

 already existing sexual and asexual forms of the Thallophytes. 



The sudden appearance of something completely new in the 

 life-history, as required by the antithetic theory, has, to my 

 mind, a certain improbability. E.x nihilo nihil fit. We are 

 iKit accustomed in natural history to see brand-new structures 



NO. 1405, VOL. 54] 



appearing, like morphological Melchizedeks, without father or 

 mother. Nature is conservative, and when a new organ is to be 

 formed it is, as every one knows, almost always fashioned out of 

 some pre-existing organ. Hence I feel a certain difficulty in 

 accepting the doctrine of the appearance of an intercalated 

 sporophyte by a kind of special creation. 



We can have no direct knowledge of the origin of the 

 sporophyte in the Bryophyta themselves, for the stages, whatever 

 they may have been, are hopelessly lost. In some of the Algce, 

 how-ever, we find what most botanists recognise as at least a 

 parallel development, even if not phylogenetically identical. (See 

 Bower, " Antithetic Alternation," p. 361). In GEdogoninm, for 

 example, the oospore does not at once germinate into a new 

 plant, but divides up into four active zoospores, which swim 

 about and then germinate. In Coleoch,ele the oospore actually 

 becomes partitioned up by cell-walls into a little mass of tissue, 

 each cell of which then gives rise to a zoospore. 



In both these genera (and many more might be added) the 

 cell formation in the germinating oospore has been generally 

 regarded as representing the formation of a rudimentary 

 sporophyte generation. If we are to apply the antithetic 

 theory of alternation to these cases, we must assume that the 

 zoospores produced on germination are a new formation, inter- 

 calated at this point of the life-cycle. But is this assumption 

 borne out by the facts ? I think not. In reality nothing new 

 is intercalated at all. The "zoospores" formed from the 

 oospore on germination are identical with the so-called 

 " zoogonidia," formed on the ordinary vegetative plant at all 

 stages of its growth. 



In science, as in every subject, we too easily become the slaves 

 of language. By giving things different names we do not prove 

 that the things themselves are different. In this case, for 

 example, the multiplication of terms serves, in my opinion, 

 merely to disguise the (acts. The reproductive cells produced 

 by the ordinary plant of an GEdogoninm are identical in develop- 

 ment, structure, behaviour, and germination with those produced 

 by the oospore. The term " zoogonidia " applied to the former 

 is a "question-begging epithet," for it assumes that they are not 

 homologous with the "zoospores" produced by the latter. I 

 jirefer to keep the old name zoospore for both, as they are 

 identical bodies. 



To my mind the point seems to be this. An CEdogonium (to 

 keep to this example) can form zoospores at any stage of its 

 development ; there is one particular stage, however, at which 

 they are always formed — namely, on the germination of the 

 oospore. Nothing new is intercalated, but the irregular and 

 indefinite succession of sexual and asexual acts of reproduction 

 is here tending to become regular and definite. 



In Splueroplea, as was well pointed out by the late Mr. 

 Vaizey {Annals of Botany, vol. iv. p. 373), though his view of 

 alternation was very different from that which I am now putting 

 forward, the alternation is as definite as in a moss, for here, so 

 far as we know, zoospores are only formed on the germination 

 of the fertilised ovum. If Spluvroplea stood alone we might 

 believe in the intercalation of these zoospores, as a new stage, 

 but the comparison with Ulothrix, Gidogonium, Bulbochicte and 

 Coleochate shows, I think, where they came from. 



The body formed from the oospore is called by Pringshei 

 the first neutral generation. In CEdogoniutn this has n 

 vegetative development, for the first thing that the oospore 

 does is to form the asexual zoospores, and it is completely used 

 up in the process. In other cases it is not in quite such a hurry, 

 and here the first neutral generation has time to show itself as 

 an actual plant. This is so in Ulothrix, a much more primitive 

 form than QLdogoniitm, for its sexuality is not yet completely 

 fixed. Here the zygospore actually germinates, forming a dwarf 

 plant, and in this stage passes through the dull season, producing 

 zoospores when the weather becomes more favourable. On 

 Pringsheim's view the dsvarf plant is not a new creation, but 

 just a rudimentary Ulothrix, which soon passes on to spore- 

 formation. So, too, with the cellular body formed on the 

 germination of the oospore of Coleoclucte : this also is looked 

 upon as a reduced form of thallus. On any view this genus is 

 especially interesting, for the sporophyte remains enclosed by 

 the tissue of the sexual generation, thus offering a striking 

 analogy with the Bryophyta. 



In the Phycomycetous Fungi — plants which have lost their 

 chlorophyll, but which otherwise in many cases scarcely differ 

 from Alg;e — the oospore in one and the same species may 

 either form a normal mycelium, or a rudimentary mycelium 



