PRESIDENTIAL ADDRESS. 713 



the specific substance of a fern can, as it were, exist in two anotropic modifica- 

 tions, the properties of which are revealed in the unlikeness of the two genera- 

 tions. Presented thus, alternation of generations becomes a special aspect of the 

 problem of individual development. 



This aspect of the problem is, indeed, apparent within what we naturally 

 regard as individual development, whenever this is discontinuous. Thus in the 

 moss we see within the limits of the sexual generation a marked discontinuoiis 

 development leading to the formation of the leafy shoots upon the protonema. 

 The change to the more complex type of plant-body is in some way determined 

 in a single cell, which proceeds to develop in a new fashion. In some cases the 

 stimulus of light of sufficient intensity appears to be effective, but we know 

 nothing of the internal factors at work. The change, however, is not rigidly 

 pre-determined, and it is of interest to find it at first reversible ; the apical cell 

 of the young bud may continue as a protonemal filament, though this rarely 

 occurs in an older shoot. As another instance of discontinuous development, 

 where also we have hints of an explanation, I may remind you of the in- 

 florescences of Veronica Chamadrys. They differ in a number of respects from 

 the main vegetative shoot of the plant, for instance in having spiral instead of 

 decussate leaves and a different type of hair. When Klebs,'" experimenting on 

 the transformation of reproductive to vegetative shoots, succeeded with in- 

 florescence cuttings of Veronica, the unknown modification in the growing point 

 resulted in a change to the whole alternative system of relations, the growth 

 continuing as a vegetative shoot with decussate leaves. These two examples 

 suggest that from the causal point of view the alternation of shoots and the 

 alternation of bionts again become parts of the same problem. There is no 

 sharp line between continuous and discontinuous development, but the dis- 

 continuity makes it easier to analyse, and perhaps experimentally attack, some 

 problems of development. 



This last consideration applies to the normal alternation of the prothallus 

 and fern-plant ; the new start from the fertilised egg gives an impression of 

 abrupt discontinuity greater than that in the origin of the inflorescence of 

 Veronica or the moss-bud on the protonema, but the cases are not essentially 

 different. It is true that the egg at fertilisation appears as a separate little 

 mass of substance in the venter of the archegonium, but after fertilisation there 

 is the closest physiological connection between the prothallus and embryo. In 

 a sense the latter behaves as if it were a special bud or branch of the prothallus. 



Some years ago I attempted to re-state from the ontogenetic side the ques- 

 tion of the different development of the enclosed egg-cell from the free spore. 

 I assumed the two germ-cells to be ' essentially alike, the different products 

 of their development depending on the different conditions.' The development 

 of the zygote in relation to the enclosing gametophyte was regarded as the 

 important factor. Though the position taken up was somewhat crude, it was 

 useful in eliciting statements on the subject from a number of botanists ; this 

 response being more valuable than the stimulus. The view advanced has been 

 criticised in most helpful fashion by Professor V. H. Blackman,'' who took the 

 ontogenetic ground to which I was endeavouring to shift ihe problem. He 

 considers the egg and spore to be different, in that 'one has received from 

 the plant which bore it a tendency to become a sporophyte, the other a tendency 

 to become a gametophyte.' He further extends the' idea of correlation, as 

 explaining the orderly development of an individual, to the whole life-cycle, and 

 considers ' the various stages as united together by a cyclical correlation, one 

 stage influencing the development of the other.' 



Professor Blackman's view does not seem inconsistent with mine, but together 

 with it gives a better statement of the position. My attempt was really towards 

 an explanation of this cyclical correlation. When germ-cells are separated from 

 the parent body, as in the case of the various spores of a Uredineous fungus, 

 any differences in their powers must have been impressed on them previously 

 and are manifested given the proper conditions. So far as this goes, it applies 

 to the spore of the fern and in part to the egg. But the latter is in a different 



" Willkurliche Entwickelungsdnderungen bei Pflanzen, p. 69. 

 " New Phytologisf, vol. viii. p. 207. 



