996 REPORT—1896. 
animals, an agreement which is conspicuously manifest in those special divisions 
which take place during the maturation of the sexual cells. Is this striking agree- 
ment the product of inheritance from common ancestors, or is the parallelism 
dependent solely on similar physical conditions in the cells? This is one of the 
great questions upon which we may hope for new light from the histological dis- 
cussion next week, 
ALTERNATION OF GENERATIONS, 
We have known ever since the great discoveries of Hofmeister that the develop- 
ment of a large part of the vegetable kingdom involves a regular alternation of 
two distinct generations, the one, which is sexual, being constantly succeeded—so far 
as the normal cycle is concerned—by the other which is asexual. This alternation 
is most marked in the mosses and ferns, taking these words in their widest sense, — 
as used by Professor Campbell in his recent excellent book. In the Bryophyta, 
the ordinary moss or liverwort plant is the sexual generation, producing the ovum, 
which, when fertilised, gives rise to the moss-fruit, which here alone represents the 
asexual stage. The latter forms spores from which the sexual plant is again 
developed. 
In the Pteridophyta the alternation is equally regular, but the relative develop- 
ment of the two generations is totally different, the sexual form being the insigni- 
ficant prothallus, while the whole fern-plant, as we ordinarily know it, is the 
asexual generation. 
The thallus of some of the lower Bryophyta is quite comparable with the pro- 
thallus of a fern, so as regards the sexual generation there is no difficulty in seeing 
the relation of the two classes; but when we come to the asexual generation or 
sporophyte the case is totally different. There is no appreciable resemblance 
between the fruit of any of the Bryophyta and the plant of any vascular 
Cryptogam, 
There is thus a great gap within the Archegoniate ; there is another at the 
base of the series, for the regular alternation of the Bryophyta is missing in the 
Algz and Fungi, and the question as to what corresponds among these lower 
groups to the sporophyte and odphyte of the higher Cryptogams is still disputed. 
Now as reyards this life-cycle, which is characteristic of all plants higher than | 
Alge and Fungi, there are two great questions at present open. The one is 
general: are the two generations, the sporophyte and the odphyte, 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 Archegoniatz 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. Professor Bower, its chief exponent, says:1 ‘It will also be 
assumed that, whatever may have been the circumstances which led to it, anti- 
thetic alternation was brought about by elaboration of the zygote [2.e. the fertilised 
ovum] so as to form a new generation (the sporophyte) interpolated between suc- 
cessive 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,’ the author describes the hy po- 
thetical first appearance of the sporophyte as follows: ‘Once fertilised, a zygote 
might in these plants [the first land plants] divide up into a number of portions 
ceamposperers each of which would then serve as a starting-point of a new indi- 
vidual. 
1 «Spore-producing Members,’ Phil. Trans. vol. clxxxv. B. (1894) p, 473. 
2 Annals of Botany, vol. iv. (1890), p. 362. 
