91 



" As the season advances this change becomes more ap- 

 parent. The process of bark increases perceptibly till it 

 reaches the fibro -vascular bundles, when it receives a slight 

 check, but soon continues its progress until these are nearly 

 eaten through or absorbed, and the prolongation of the epider- 

 mis has entirely covered the surface of the articulation. At 

 the same time, (iu the poplar very distinctly, and in other 

 plants more obscurely,) a great change is taking place in the 

 contents of the cells, on both sides of this prolongation, in the 

 formation of a large quantity of starch, probably for the nutri- 

 tion of the young leaf-bud when it begins to expand next year 

 In the poplar this deposit of starch takes place in the bark 

 and wood at the base of the leaves, to a great extent, so much 

 so, that iodine renders a section completely dark; and not only 

 at tins spot, but for a very short distance also on the distal 

 side of the joint. 



"If we now make a separation of the base of the petiole 

 and the stem under the microscope, we shall find that the dis- 

 ruption takes place invariably through the cellular tissue 

 external to the prolongation of the epidermis, so that when it 

 has been effected, the tree does not suffer from the effects of 

 an open wound. 



"The change that takes place in the cellular tissue prior to 

 its disruption, appears to me to be simply a sort of self-disin- 

 tegration; the cells contract and become rounder, and separate 

 their walls from each other, so as to destroy their cohesion 

 1 am very greatly inclined to think, from the generation of 

 starch in their interior, that the process is altogether a vital 

 one; that it is, in fact, the last act of life at the base of the 

 petiole. We are strengthened in the belief of the vital charac- 

 ter of the act, when we consider that the provision for the fall 

 of tli, leaf; the formation of the articulation, has been going 

 ou from the earHest existence of the petiole, that it advances 

 Wiethe growth of the leaf, and is not complete until the leaf 

 itseli is ol no more use. 



