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CHAPTER XIII. 



THE EFFECTS OF STEAINS ON STRUCTURES. 



Vegetative Organs. — In explaining the origin of iiTCgulur 

 flowers by insect agency, it will not be amiss to fortify the 

 theory by describing other instances apart from flowers, and 

 to add further results which I believe to accrue from the 

 persistent action of insects on the one hand, and a ready 

 response on the part of the organ on the other. 



Researches into the anatomy of stems have proved the 

 existence of this responsive power. Thus, a tree will develop 

 wood in a particular direction if it be compelled to meet 

 special strains imposed upon it ; for Andrew Knight found 

 that when trees were allowed freedom in one direction only, 

 and were thus made to oscillate in definite directions, either 

 east and west or north and south, the stem became elliptical 

 in section, the long axis corresponding to the direction of 

 oscillation. Mr. Herbert Spencer has also described how 

 Cactuses, if submitted to particular strains, develop wood to 

 meet them. 



The various kinds of the supporting tissues of pedicels, 

 such as collenchyma, sclerenchyma, the so-called liber-fibres 

 as well as true woody fibre, are all so many contrivances of 

 the stems to support the weight of the flowers and fruits, and 

 to overcome gravity. So, again, in the case of apples and 

 pears, if they hang vertically downwards they grow as 



