FALL OF THE LEAF. 163 



season, but generally it is not far advanced in development until 

 near the end of summer. The leaflets of the larger compound 

 leaves for instance, those of Ailanthus, Gymnocladus, Ju- 

 glans, etc. afford excellent material for examining the process 

 of defoliation. Strong leaves of any of the plants mentioned 

 are to be kept between damp (not wet) paper in a warm place 

 for a number of hours, when the formation of the dividing plane 

 can be observed. The plane is so far completed by the end of 

 the second or third day that the leaflets fall with the slightest 

 touch. 



459. The strong leaves of horse-chestnut are employed by 

 Strasburger as material for demonstrating the process of defolia- 

 tion. He says that alcoholic material answers very well for the 

 purpose, but that it happens occasionally that the distinctive 

 brown color of the cells adjoining the cutting plane is nearly or 

 quite lost. The petiole is to be cut through in its median line, 

 and then several very thin longitudinal sections parallel to this 

 are to be carefully made and placed at once in water. In a good 

 preparation the cells making up the cutting plane should be 

 clearl}' seen extending from the epidermis of the petiole to the 

 fibro-vascular bundles. If the leaf was taken at just the right 

 time, the preparation should show also that the cutting plane 

 has invaded even the tissue of the fibro-vascular bundles. The 

 plane consists of one to several layers of cells, some of which 

 are plainly cutinized ; thus, as a rule, the place of separation is a 

 scar healed before the leaf falls. 



It happens frequently that changes take place at the middle 

 portion of the cutting plane, by which its layers near the leaf 

 are forcibly separated from those nearer the stem ; in such cases 

 the leaf falls because it is forced off. 1 



460. The excision of the leaf usually takes place at the base 

 of the petiole, so that the surface of the scar is even with the 



1 "The provision for the separation being once complete, it requires little 

 to effect it ; a desiccation of one side of the leaf-stalk, by causing an effort of 

 torsion, will readily break through the small remains of the fibro-vascular bun- 

 dles ; or the increased size of the coming leaf-bud will snap them ; or, if these 

 causes are not in operation, a gust of wind, a heavy shower, or even the 

 simple weight of the lamina, will be enough to disrupt the small connections 

 and send the suicidal member to its grave. Such is the history of the fall of 

 the leaf. We have found that it is not an accidental occurrence, arising simply 

 from the vicissitudes of temperature and the like, but a regular and vital pro- 

 cess, which commences with the first formation of the organ, and is completed 

 only when that is no longer useful" (Dr. Inman, in Henfrey's Botanical 

 Gazette, vol. i. p. 61). 



