METAMORPHOSIS IN PLANTS. 103 



Whilst then we admit that external conditions and 

 agencies may be the vera causa of any given metamorphosis, 

 we cannot fail to perceive that they constitute but the proxi- 

 mate cause, whilst the ultimate cause resides in the organisa- 

 tion of the plant itself. This has long been recognised, and 

 a plastic force or vis formativa?- whereby it develops the 

 characteristic form of its organs, has been attributed to the 

 living organism. I need hardly point out, however, that 

 such an assumption as this is no explanation of the pheno- 

 mena ; it is merely a statement, in abstract form, of the 

 facts of development. It is obviously true, but it is no more 

 than a truism beyond which we must pass if we would attain 

 to a knowledge of the physical conditions which are the ulti- 

 mate causes of the phenomena. The only attempt in this 

 direction with which I am acquainted is that of von Sachs 2 

 — whose recent loss all Botanists have to deplore — who has 

 suggested that the development of the different kinds of 

 organs is the result of the elaboration of certain specific plastic 

 materials in the plant, a view to which Goebel also adheres 

 to in the main. 3 But this suggestion has not been sufficiently 

 worked out to warrant its acceptance. And even were it 

 proved to be true, the vis formativa would not have been 

 explained away, for the questions would still remain : why 

 and how does any particular plant produce just those sub- 

 stances which are necessary for the development of its own 

 peculiar forms of leaf, flower and fruit ; and how is the appro- 

 priate distribution of these substances in the plant effected 

 at just the right time and in proportionate quantity ? We 

 must admit that Science has not yet lifted the veil which 

 enshrouds this last mystery of metamorphosis. 



I have now concluded my task. I have laid before you 

 to the best of my ability the history of the origin and 

 growth of the doctrine of metamorphosis. With regard to 



1 Blumenbach's Nisus formativus (Ueb. den Bildungstrieb und das 

 Zeugungsgeschafte, 1781): see Vochting, Ueb. Organbildung im Pflanzen- 

 reich, i., 1878. 



2 Von Sachs, " Stoff und Form der Pflanzenorgane," Arb. d. bot. Inst, in 

 Wiirzburg, ii., 1882. 



3 Goebel, Vergleichende Entwickelungsgeschichte, p. 113. 



