CiTAr. YI] 



ANIMALS 



143 



an indubitable adaptation is exhibited. Above the insertion of every leaf 

 there runs nearly up to the next node a shallow groove, the summit 

 of which displays a roundish 

 depression, both in non-myrmeco- 

 philous trees and in young inter- 

 nodes that are not yet inhabited 

 (Fig. 7S), As the external de- 

 pression corresponds to an in- 

 ternal one, the wall at this place 

 is very thin and is a mere 

 diaphragm in a tube (Fig. 79). 

 The diaphragm differs essentially 

 in its histological structure from 

 the adjoining parts of the wall, 

 for it is devoid of the hard and 

 tough elements, such as vascular 

 bundles, collenchyma, lignified 

 parenchyma, which form the main 

 mass of the tissues in all other 

 places. In the tissue of the wall 

 underneath the groove thevascular 

 bundles arise as secondary struc- 

 tures, and stop short just beneath 

 :he diaphragm. The diaphragm 

 s destined to be the entrance ; 

 ^nd the wall is bored through 

 ilways at this spot. 



Investigations into the history 

 jf the development show that 

 it first the depression arises as 



result of the pressure exerted 

 3y the little axillary bud, which 

 visible in the accompanying 

 igure (Fig. 78) at the base of 

 he internode. This pressure is 

 :xerted during the whole longi- 

 udinal growth of the internode 

 nd causes the formation of the 

 roove. The tissues internal to 

 he groove resemble those of the 

 •arts that are not exposed to 

 ressure except the original little 

 Iter the pressure has ceased and at the same time is further modified. 



Fig. 78. Cecropia adenopus. Summit of a young 

 stem. On one internode the place of entrance a is 

 not yet bored through ; on another internode the 

 entrance i is bored through. Natural size. 



pit, which increases in circumference 



