LECTURE XXXI. 



INFLUENCES OF THE ENVIRONMENT ON THE PROCESSES OF 

 CONFIGURATION IN THE PLANT. 



We are now acquainted with a considferable number of eases where, by 

 means of external influences, and particularly those of Gravitation, Light, Pressure, 

 and by stimuli due to animals or plants, processes of g-rowth are modified or are 

 even induced ; this however not simply in the sense of accelerating or retarding 

 the increase in volume — on the contrary, I have here in view the processes 

 of configuration itself, the production of new growing points and their embryonic 

 outgrowths, and further extensive qualitative alterations of organs already existing 

 in a young state, by means of external influences. 



The processes to be described have to be regarded as phenomena of irritability 

 in the growing parts of plants; since, as has already been incidentally men- 

 tioned, phenomena of irritability in general are fundamentally nothing but specifi- 

 cally peculiar reactions of the organism towards external stimuli, and it is just 

 the cases which are to be considered here which belong to ttiis category. The 

 province of the phenomena of irritability, however, is so extensive that, properly 

 put, the whole of physiology is everywhere and always concerned with it, so 

 that in fact physiology might simply be designated the science of the phenomena 

 of irritability. But it is exactly on this account that I wish to bring forward the facts 

 to be described here particularly and by themselves, apart from the endless number 

 of phenomena of irritability, and not to put them off to the subsequent and more 

 detailed consideration of the induced movements, because they contribute in an 

 important manner to render our insight into the nature of the growth of plants more 

 profound. 



Until quite recently the opinion has been retained, dating from times imme- 

 morial in the period of the development of human thought, that organic forms are 

 something fixed and unalterable from all eternity, and thus removed from the region 

 of causality ; and with reference to this mode of viewing the matter it is probably 

 worth while to convince ourselves, by taking a few well marked cases more 

 closely into consideration, of the fact that incidental external stimuli are able to 

 aid in determining the processes of configuration in organisms in very definite 

 ways. This old mode of representing matters is- of course now opposed by 



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