homologous Alternation of Generations in Plants. 369 
then the position adopted by me in a previous article in this 
journal is the natural consequence 1 . I have there insisted 
upon the conclusion that the axis and leaf of the gametophyte 
(i.e. in the Moss) are not the true homologues by descent of 
the axis and leaf in the sporophyte (e.g. Fern or Lycopod) : 
both are doubtless similar from the physiological point of 
view, for both are to be regarded as a means of enlargement 
of surface, and of its exposure to air and light, in order that 
nutrition may the more freely go on ; but in discussing their 
nature, and in classifying such parts, we are bound to take 
a general rather than a one-sided view ; and while recognising 
the external form and physiological significance of the leaf, it is 
necessary also to take into consideration its origin by descent : 
once concede that the alternation in the archegoniate series is 
by interpolation of a new stage — the sporophyte — and it must 
necessarily follow that this, the newer generation, cannot be 
the result of a mere transformation of the old one, and con- 
sequently also the parts of the pre-existent generation — the 
gametophyte — cannot be strictly comparable to the parts of 
the interpolated generation, that is, of the sporophyte. The 
foliar differentiation must have taken place in the two quite 
independently, though as a similar response to the needs and 
external conditions of the plants. I have suggested that this 
point should be indicated in the terms used, and that while the 
axis and leaf of the sporophyte are styled the true caulome 
and phyllome, the terms phyllidium and caulidium should be 
applied to the correspondingly differentiated parts of the game- 
tophyte. This suggestion was made, not as a mere effort of 
terminology, but rather as indicating a distinction of the first 
importance as regards the history of evolution of the main 
series of plants. I am disposed to think that, as our know- 
ledge becomes more assured, it may be found necessary to 
subject the gametophyte and the sporophyte to an entirely 
distinct and separate morphological treatment, notwithstanding 
the many points of analogy between them as regards the form 
1 Annals of Botan}', vol. i. p. 133. 
B b - 2 , 
