INTRODUCTION 145 



In addition to the varying behaviour of the juvenile form towards 

 the external conditions of plant-life, there is another factor to be con- 

 sidered which finds special expression in the configuration of the leaves of 

 the juvenile stage. The difference in the configuration of the juvenile leaves 

 compared with that of the adult ones is frequently due to the fact that 

 they are arrested formations ; in other words, the development of the 

 leaves is the same in both juvenile and adult, but in the juvenile the 

 primordium of the leaf is arrested in its development at a certain stage 

 and therefore the leaf exhibits an evident, often extremely, different 

 configuration. This point in the history of development must also be 

 applied to the explanation of the differences between the configuration 

 of those juvenile forms which have been already referred to as phylo- 

 genetically primitive and the adult forms, inasmuch as the latter have 

 acquired their different character by passing through a further trans- 

 formation. 



In many plants reversion of the adult to the juvenile form frequently 

 occurs. This will be specially dealt with in subsequent pages. 



What follows does not profess to be a comprehensive and systematic 

 account of the development of the different plant-forms, but in accordance 

 with the aim of general organography an attempt has been made to 

 select from the different groups of plants examples which shall exhibit 

 the characteristic configuration of the juvenile form, and to place them 

 before the reader in such a way as shall best bring into prominence the 

 different sides of the problem. The configuration of the cotyledons in 

 the Spermaphyta is therefore entirely passed over here, that will be spoken 

 of in the special part of this book ; I need only say here that the coty- 

 ledons which so frequently differ in form from the foliage-leaves are 

 merely arrested forms of these, the arrest being sometimes permanent, 

 sometimes transient. The cotyledons of Ampelopsis furnish examples 

 of the latter ; they are originally small and simple, but after germination 

 they grow into relatively large foliage-leaves. Even more striking are 

 the phenomena observed in many species of Oenothera *, where, by inter- 

 calary growth at the base of each cotyledon after germination, a portion 

 is interposed which is much larger than the original cotyledon. In these 

 plants then the arrest in the development of the cotyledons lasted only 

 during the period of rest in the seed and the first stages of germination, 

 and the difference between this and what happens in most other cotyledons 

 is, in my opinion, only quantitative, not qualitative. 



The duration of the juvenile form is scarcely less variable than its 

 external configuration, and is frequently dependent upon external factors, 

 especially in lower plants. In some of these the juvenile form is the 



1 See Lubbock, A contribution to our knowledge of seedlings, i. p. 553. London, 1892. 



GOEBEL L 



