Chap. VI] 



ANIMALS 



H3 



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. 78). 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 

 the diaphragm. The diaphragm 

 is destined to be the entrance ; 

 and the wall is bored through 

 always at this spot. 



Investigations into the history 

 of the development show that 

 at first the depression arises as 

 a result of the pressure exerted 

 by the little axillary bud, which 

 is visible in the accompanying 

 figure (Fig. 78) at the base of 

 the internode. This pressure is 

 exerted during the whole longi- 

 tudinal growth of the internode 

 and causes the formation of the 

 groove. The tissues internal to 

 the groove resemble those of the 

 parts that are not exposed to 



pressure except the original little pit, which increases in circumference 

 after 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 b is bored through. Natural size. 



