274 



a considerable number of examples of the relation between form, blooming time, and external conditions. 

 These he presents and discusses in his usual luminous fashion. He has even cited briefly analogous 

 phenomena, not a few from the animal kingdom. 



The thesis of the book is that the generative maturity of plants is not connected immutably with 

 a definitive stage of their development, as has been so widely held. A certain minimum of nutritive prepara- 

 tion is presupposed ; but once this is passed, there is a broad variation zone in which blooming occurs. Its 

 apjsearance is dependent upon complex, largely unknown conditions, an important part of which, however, 

 are external. The vegetative ontogeny depends upon the co-operation of autogenous and exogenous (an 

 excellent substitute for the awkward term " aitiogenous ") factors; for the rudiments of the vegetative 

 organs have many possibilities, and which one is realised is determined by the environment. The mature 

 form of the entire organism is thus a product of vegetative ontogeny and of generative maturity, both of 

 which factors are variable, though their variability is not in the same direction. True, the development 

 of vegetative structures usually ceases at blooming, but this is the only place where the two lines of develop- 

 ment, the vegetative and the generative, are inseparably connected. Elsewhere they are free and inde- 

 pendent of one another, and eaeh varies after its own manner. In this connection of two variable factors 

 lies an important impulse to increase the - manifold forms of the plant world. For the conditions which 

 help to regulate the succession of leaf forms and floral maturity change with the changes of climate in space 

 and in time, giving rise to local geographic species and allowing true species to arise in the course of time. 

 Their features attain heritability, and become therewith a source of new lines with new possibilities. 



A new term. " helicomorphy," is suggested to comprehend Goebel's two terms for the two-leaf forms 

 in heteroblastic species, the juvenile forms and successive forms. In the course of a short chapter on the 

 phylogenetic significance of helicomorphy, the author pays his respects to the famous " biogenetic law," 

 that ontogeny recapitulates phylogeny, in these terms : " In the botanical field it has absolutely no (nicht 

 einmal immer) heuristic value, and whoever allows himself to be led by it will at most succeed in satisfying 

 the needs of his imagination."' 



In the genus Eucalyptus we are accustomed to see — ■ 



1. Plants flowering when in the mature lanceolate-leaved stage. The juvenile 



leaves may be, and usually are, of a different shape. This is heteroblasticity. 



2. In a few cases the leaves maintain their juvenile form through life. (Isoblasticity 



or homoblasticitv.) 



3. In a number of cases, and careful observation increases the number from time 

 to time, Ave find plants which normally fall under (1), flower as regards individual 

 branches, while in the juvenile stage. 



At p. 97 of Dr. Diels' work already referred to, he says (translation) : — 



Everywhere in Eucalypts are shown close relations between juvenile forms and flowering maturity, 

 lb will be a very useful work for the Au?tralian botanists to add new facts by observation in the field and 

 in cultivation. Thereby it will perhaps also be possible to find out the conditions, of which we know very 

 little at present. There is at least one fact which manifests itself empirically : Tin number of forms which 

 flower while their foliage is in the juvenile state is specially numerous in regions ivheretlie surrounding medium 

 is al a considerable distance from the optimum of the genus. (N.B. — -I have dealt with the question of Optimum 

 at Part 69 of my " Forest Flora of New South Wales/' — J.H.M.). The cool regions of Tasmania are rich 

 in such forms. The dry plains of the North Australian sandstone tableland possess such species, and they 

 are also found in the dry heaths of south-west Australia, which follow the interior border of the winter-rain 

 region. 



At. p. 17 he says : — 



The reigning Australian genus Eucalyptus is marvellously elastic in the condition of growth and 

 flowers. The most important instances will be given later (p. 88-98) ; for the present I will mention only 

 a f w cases given by Mueller, and from my personal observations in South- West Australia. 



