712 TRANSACTIONS OF SECTION K. 



seems impossible to overlook a segmental construction of the whole shoot in 

 relation to the leaves. 



The view of the general nature of the shoot to which we were led by a wide 

 comparison including Algae, Bryophyta, and Pteridophyta seems to enable us 

 to understand in natural relation to one another the reality of the monopodial 

 axis, its segmental construction, and the origin of the free portion of the leaves 

 as outgrowths. The shoot is a new whole to be understood in the light of the 

 method of elaboration of the single filament with subordinated appendages. As 

 soon as the shoot is recognisable as an entity, the distinction can be drawn 

 between these strictly subordinated appendages or ' leaves ' and branches which 

 repeat the whole construction. I cannot enter into the question of whether the 

 fern-leaf is to be regarded as a transf oi-med branch system, further than to state 

 my opinion that the correspondence of the fern shoot as a whole with that 

 of a Lycopod or a moss makes such a difference between their leaves unlikely." 

 The evidence in its support is drawn partly from an assumed origin of shoots from 

 a hypothetical thalloid or pro-hepatic type and partly from vascular anatomy. 

 The evidence on such questions from anatomy is, for reasons already given, 

 perhaps the least satisfactory available, since the vascular tracts are established 

 after the shoot as a whole has come into existence. They may to some extent 

 reflect the original construction of the shoot, but may be largely independent of 

 this. 



If we test the opposed theories by their application to such a fern as 

 C'eratopteris in the light of its embryology and apical development we find that 

 the conception of leaf arising from leaf without the existence of a stem apex is 

 not really supported by the facts. On the other hand, the segmentation of 

 the shoot is evident throughout, owing to its coincidence with the apical segmen- 

 tation. It is further most instructive to try to apply the various theories, 

 whether phytonic or strobiloid, to the shoot of the moss with all its parallel 

 correspondences to that of a higher plant. 



Alternation of Generations. 



The question of individual development led to the consideration of the 

 morphology of the shoot. It also naturally leads us to glance again at the old 

 problem of alternation of generations, for it is of profound interest to causal 

 morphology that two very distinct individual forms should appear in the same 

 life-cycle. The question thus rai.sed concerns the nature of alternation, and is 

 distinct from the historical question as to the origin of alternation, which for a 

 time was regarded as constituting the whole problem. Any solution of the 

 historical question depends on a knowledge of the lines of descent that have 

 Jed to the various groups of the Algae, Bryophyta, and Pteridophyta. We have 

 seen how little prospect there is of this The various examples "of alternation 

 seem best regarded as homologies of organisation, independent manifestations 

 of a similar condition. They are not less but more valuable on that account in 

 studying the nature of alternation. To find the common ground from which the 

 correspondences in this respect in the life-histories of Algae, Bryophyta, and 

 Pteridophyta result we might have to go very far back, possibly to the life-cycle 

 of unicellular organisms. Even there the phenomena might be of independent 

 origin and the homologies not homogenies. Were this the case we should have 

 to contemplate an intercalation theory of the vegetative tissues not only for the 

 .^porophyte but for the gametophyte. This is necessarily mere speculation, but 

 it helps us to realise, as Dr. Scott once said, how insoluble the problem really is. ■ 



I wish to dwell for a little on an aspect of it which is more open to attack, 

 however difficult. This is the analysis of the phenomenon as we see it in the 

 life-history of the fern with the object of arriving at some idea of the causal 

 factors in the difference between the two generations. In one sense the fern- 

 plant and the prothallus appear like two individuals ; in another the two stages 

 are like parts of the same individual. We seem almost forced to assume that 



*■ The cases in Algae where a shoot appears to result from the subordination 

 of branches to an axis are of great interest, but require further elucidation 

 themselves, while the other line of elaboratign is clear. The difficult cases are 

 jpainly found in the Fucaceae, 



