SECOND PART. 



Physiology of Growth 



and Movements Resulting 



from Irritability. 



FOURIH SECTION. 

 Movements of Growth. 



I. CHARACTERISTICS OF GROWING PLANT STRUC- 

 TURES, AND MOVEMENTS OF GROWTH DEPEN- 

 DENT ON INTERNAL CAUSES. 



146. The Extensibility and Elasticity of Growing Plant 

 Structures. 



IT is of great significance, in connection with the theory of growth, 

 that growing plant structures are highly extensible and elastic. 

 We shall return to this again in detail ; here it is only proposed 

 to establish the fact in a general way. 1 



For investigation we select perfectly fresh pieces of stem cut 

 from plants of Aristolochia Sipho or Sambucus' nigra. At the 

 upper and lower end of a young iiiternode and of the next older 

 one we make fine lines with Indian ink, as points of reference, 

 take the object with both hands, and, laying it against a milli- 

 metre scale, stretch it as strongly as may be without risk of 

 breaking it. It is now easy to prove that the younger internodes 

 are much more extensible than the older ; and I found, e.g., that 

 the extensibility of a young internode of Aristolochia Sipho, 50 

 mm. long, amounted to 9 per cent. If we leave the shoots to 



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