13 



developmental forms becomes realized.' Tliis regulation by means 

 of the environment is clearly realized in the case of heteroblastic 

 ontogenesis. 



" As in the case of flowering, the insight into the associations 

 which we have hitherto obtained is rather limited. But we see 

 that the organization of subsequent leaves [Folgebldtter) becomes 

 more abundant if heat and moisture are increased. And we 

 recognize a restriction in this respect by shortening the growth 

 period, by dryness, and by a lowering of the temperature. 



"Thus ontogeny varies with the quality and degree of ex- 

 ternal factors. The finished figure of the organism is the product 

 of vegetative ontogeny and of flower-maturity : and both factors 

 are variable. 



"And furthermore their variability is not of the same kind 

 nor similarly ordered. Of course the vegetative form-develop- 

 ment ceases for the most part with flowering ; but that is a stage, 

 however, where the two courses of development, the vegetative 

 and the generative, are indissolubly connected. Otherwise they 

 are free and independent of each other. Their relation is capable 

 of every variation. The leaf-succession in its phase (heliko- 

 morphy) varies after its own fashion. Flowering varies in its 

 own way. 



" In this combination of two variable factors into the unity of 

 the flowering form, lies a powerful impetus to the increase of 

 form-diversity in the plant kingdom. For the circumstances 

 that help to regulate the leaf-succession and flower-maturity vary 

 with the change of climate in time and place. In their ultimate 

 effects they produce the geographically local races (^Arten) and 

 in the course of time favor the development of new species. 

 Their product attains to heritability, and thereby new strains 

 with new possibilities become established. 



" In this relation of consecutive vegetative stages and flower- 

 ing, it is expressed with clear emphasis how endlessly variable 

 form is in the plant kingdom. Even the few external factors 

 that we perceive, produce an interminable maze of possibilities. 

 We are led indeed to the confession to which Klebs was led in 

 another connection: 'The typical or customary development 



