420 University of California Puhlications in Botany [Vol.6 



In young material there is no histological distinction in the cells 

 which would enable one to fix upon the ten to eighteen layers destined 

 to function later as the abscission zone. The cells are of the same 

 average size and appearance as those on both sides of the zone, and 

 prior to the initiation of the process we have as yet no way of accu- 

 rately determining its location. Evidence of a cytological nature will 

 be submitted below to show that subsequent to the application of the 

 stimulus which is to cause abscission changes occur in the cell walls 

 of the abscission zone. These changes are such that they can be de- 

 tected by optical and chemical means and constitute further proof 

 that this zone is composed of cells physiologically different from those 

 on either side. 



In older material, both at the base of the terminal leaflet and at 

 the base of the petiole, the cells in the abscission zone are smaller and 

 differ in general appearance from those on either side. They are 

 characterized by their small size, isodiametric shape and absence of 

 visible vacuoles ; they are darker in color, and, further, are somewhat 

 collenchymatous in appearance. On both sides of the abscission zone 

 the cells grade up in size, becoming lighter in color and more vacuolate 

 until they are several times as large as the cells of the abscission zone 

 proper. At the base of the terminal leaflet the cells of the abscission 

 zone are somewhat smaller, thicker walled, and darker in color than 

 those at the base of the petiole. 



The Separation Layer 

 Actual separation occurs one or two tiers of cells proximal to the 

 distal end of the abscission zone, that is, one or two layers below the 

 upper end of this zone. Ordinarily only one or two layers of cells 

 are involved in the separation process, but occasionally as many as 

 four or five may take part. However, it is seldom that the cells of 

 more than one tier actually complete the process. This is usually the 

 uppermost tier, but the process also frequently includes a portion of 

 the next lowest tier. Separation does not necessarily occur in a plane 

 straight across the petiole at right angles to the main axis, but follows 

 a more or less irregular path, undoubtedly depending somewhat on 

 the mechanical resistance encountered in the older elements and vas- 

 cular bundles. Indeed, in some cases the path is completely changed 

 in direction, paralleling the vascular elements for a short distance 

 before crossing them. The more or less jagged ends presented by the 



