174 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



out of the ground ; but the facts which have been stated show that in 

 the reversion to the juvenile form conformability with purpose cannot be 

 considered as the determining factor. There is a number of plants 

 endowed with this capacity on the advent of definite external conditions, 

 and they react thereto in the same way as does Myriophyllum when it 

 forms winter-buds upon the withdrawal of nutrition l reaction which may 

 have a definite aim but this is not always necessary. 



CONCLUSION OF THE DEVELOPMENT. 



An account of the relationships of configuration which follow the 

 juvenile state will be our task in the special part of this book. Here 

 I have only briefly to refer to the question of how far we can speak in 

 plants of adult features. The formation of the organs of propagation, 

 especially of the sexual organs, marks a climax of the development. It 

 causes in many cases the closure of vegetative development, but in others 

 it does not do so. The changes in configuration which precede the 

 formation of the propagative organs, for instance the formation of bracts 

 in the region of the flower, cannot be designated adult features. Where 

 vegetative shoots possess a limited development relationships of correlation 

 are chiefly concerned in it. But even where these are not proved, as for 

 instance in Schistostega, in which the terminal bud of the vegetative shoot 

 which is always unbranched loses after some time its power and dries 

 up, correlations are nevertheless probably effective. In plants which are 

 able to produce organs of propagation frequently time after time the 

 progressive increase of the vegetative framework finally brings about, as 

 has been above stated, the phenomena of ' age ' and ultimately death. 

 In Melocactus we find the converse. In it when the plant has attained 

 a certain size there is formed at its summit a ' tuft,' really an inflorescence, 

 which grows persistently for years whilst the vegetative parts neither 

 branch nor probably increase considerably in volume. Here the persistent 

 increase of the flower-bearing region of the plant evidently represents the 

 ' atrium mortis,' for the plant becomes exposed with the increasing size 

 of the tuft always more to injuries, but the vegetative capacity is small 

 and later probably entirely disappears. 



1 Goebel, Pflanzenbiologische Schilderungen, ii. p. 360. 



