PRESIDENTIAL ADDRESS. 715 



liow either external or internal influences equivalent to the usual relations can 

 come into play. There is no nuclear change, at least of the nature of nieiosis, 

 and there seems nothing for it but to assume some material modification involv- 

 ing the change to the alternative condition of the specific substance expressed 

 in the prothallus. The interest of these considerations, tentative as they must 

 be, lies in the way in which they associate two explanations of development 

 the influenr^e of particular, unknown determining substances and the system of 

 relations. 



The whole question is fortunately not complicated with adaptation or any 

 gradual origin of these deviations from the normal, and affords a particularly 

 clear example of a problem in causal morphology. The perfect development 

 of isolated members without the usual relation to the rest of the plant-body 

 has an important regulative bearing on the current assumption that every stage 

 in development is determined by the preceding stage. In this connection a 

 most interesting parallel can be traced between the appearance of perfectly 

 formed roots, ramenta, sporangia, and vascular tissue in induced apogamy 

 and the development of bones, teeth, and hairs in dermoid cysts occurring in 

 the human ovary or testis. The further study of the conditions of development 

 in such abnormal cases may do much to enlighten us as to the factors concerned 

 in the normal ontogeny. 



The Seed and its Embryo. 



So far the problems considered in illustration of the possibilities of a 

 general causal morphology have been suggested by the fern. The nature of 

 roots, of the sexual organs, and of sporangia might also be profitably looked 

 into from the non-phyletic point of view. I shall instead step beyond the 

 fern and glance very briefly at some problems of the seed and the embryo of 

 the seed plants. The story of the great additions made of recent years to 

 our knowledge of Palteozoio seeds is familiar to us all. It remains an open 

 question, however, to what extent seed-plants are to be regarded as of poly- 

 phyletic origin and in particular to what extent their seeds are homogenous or 

 are homologies of organisation. In spite of the wonderful widening of the 

 field of comparison by the discovery and investigation of the Pteridosperms 

 we have no compelling evidence of actual lines of descent showing steps in the 

 origin of the seed. The presumption is now in favour of an origin of all or 

 most seed-plants from ancient Filicales, the earlier view of their derivation 

 from the Lycopodiales being weakened or abandoned. Since this phyletic 

 problem lies, in part at least, within the period of geological history, direct 

 evidence may be hoped for. But in the light of the progress of opinion on 

 other large questions of descent we must remember the possibility of parallel 

 evolutions, and may even suspend judgment as to whether the Cycadophyta have 

 been derived from Pteridosperms or the Angiosperms from the Bennettitales. 

 Comparisons of the organisation of the whole plant seem somewhat forced in 

 both cases. 



Looking at the facts broadly, we can hardly escape from a very strong 

 suggestion of parallel development affecting a number of distinct groups. There 

 is evidence of this in the case of heterospory, which is found at various points in 

 the Lycopodiales, Filicales, and Equisetales, usually with no indication of its 

 being a gradually acquired or an adaptive character. Heterospory is otherwise 

 .1 darker problem than the origin of the seed-habit to which it was presumably 

 the preface. The study of the variety in early seeds has suggested to experi- 

 enced investigators that seeds also may be the result of independent develop- 

 ment." 



From the point of view of causal morphology the seed appears to present 

 problems parallel to those of the enclosure of the archegoniate sporophyte in 

 the arcliegonium. The distinction made between effective and ineffective 

 retention applies here also. The fact that the megaspores of some species 

 of SekifjlneUa germinate within the sporangium is not really an approach to 

 the seed-habit ; it is ineffective retention so far as results on either the spore- 

 contents or the sporangium are concerned. In the development of the ovule, 

 however, we find the embi-yo-sac constituting with the investing tissues a new 



-" Cy. Oliver and Salisbury, Annnlx of Botany, vol. xxv. p. 46. 



