INTRODUCTION 143 



thallus of considerable circumscription, may give rise to new gemmae 1 . 

 Obviously the plant here saves the ' germ-plasm ' by the shortest possible 

 way under conditions which are unfavourable for development. But 

 commonly there is intercalated between germination and the formation of 

 the germ a long series of developmental stages which are of advantage to 

 the species because they make possible a considerable increase of the 

 plant-body and through it the formation of numerous germ-cells. 



The appearance of these germ-cells always marks a climax of develop- 

 ment, in many cases its conclusion, which is only gradually reached. The 

 first stages may be called the juvenile stages. It is of course impossible 

 to limit these sharply -. The difference between these juvenile stages 

 and the adult form may be more or less great. These two stages 

 naturally include again series of developmental processes which pass 

 one into the other without distinct limitation. The difference between 

 the two is expressed not only in configuration, but frequently also in 

 other characters particularly in the capacity for reproduction. The first 

 leaves of the germ-plant of Lycopodium inundatum, for example, possess 

 the capacity of producing adventitious shoots, but this is wanting in 

 those which are formed later 3 ; in Utricularia montana we find analogous 

 phenomena; and in many Coniferae cuttings of the juvenile form root 

 readily, whilst those of the adult form do so with difficulty or not at 

 all 4 . The juvenile form also exhibits different relationships of direction : 

 in many plants, for example, Tilia, Fagus, Carpinus 5 , whilst the seedling 

 is orthotropous the later shoots are plagiotropous ; in other plants, for 

 example, in the Marcgravieae and in the root-climbers amongst the 

 Aroideae which will be referred to later, the opposite relationship is 

 observed. This is only one of the numerous examples showing that tJie 

 adaptation of the juvenile form to external relationships is different from 

 that of the adult form, a fact which appears in a very striking way in the 

 ' larval form ' of many animals. The differences between the two sections 

 of the developmental history show themselves in very different degrees ; 

 there are cases where they are very slight and the two sections may 

 quite gradually pass one into the other. I have designated this latter 

 condition the homoblastic development, and that in which the differences 

 are great the heteroblastic. 



Amongst higher plants Casuarina may serve as an example of the 



1 Goebel, Morphologische und biologische Studien, in Annales du jardin botanique de Buitenzorg, 

 vii. p. 71. 



2 We are here concerned only with the features which appear after germination and not with the 

 arrangement of the cells and the configuration of parts which obtain, for instance, in the embryo 

 within the seed of one of the higher plants. 



3 See what is said below about Preissia commutata. 



1 See page 51. 5 See page 70. 



