372 University of California Publications in Botany [Vol.5 



the epidermis (pi. 49, fig. 2) over the tissue in which the process is 

 taking place. Simultaneously with the start of abscission in the 

 cortex, the process apparently originates independently in the pith 

 (pi. 50. fig. 1). It was further noted that the number of cells con- 

 cerned in the process, as a general rule, is greater in the hybrids than 

 in their parents and also that this is true of ' ' automatic ' ' as compared 

 with "spontaneous" abscission. Just beneath the epidermis the cells 

 involved in separation were reported as being from five to ten tiers 

 thick, but as the process approached the vascular tissue the separation 

 layer was evidently reduced in thickness to not over one or two tiers 

 of cells (pi. 52, fig. 1). In the pith a more or less spherical mass of 

 cells is involved (pi. 50, fig. 1). When the separation is completed 

 the flower may remain in position for some time, until the epidermis 

 and tracheal elements are broken by some mechanical agency. 



The exposed separation surface of the pedicel was stated to be 

 convex in outline and slightly notched at the tip. Upon closer exam- 

 ination the surface itself was seen to be composed of the protruding, 

 rounded ends of cells with here and there completely isolated cells and 

 broken ends of spiral trachea 3 . These isolated cells are apparently 

 normal and do not markedly differ in form, size, or in the nature of 

 their cell inclusions from the same cells before separation. The 

 exposed surface of the attached portion of the pedicel is similar in 

 appearance to that of the detached portion, but is more or less flat 

 in outline. After separation the cells on this surface collapse and 

 probably act as a protective layer. 



Following the observations recorded above, which had to do largely 

 with flower-fall in the F a species hybrids, a number of species have 

 been investigated in an effort to determine whether or not their mode 

 of abscission differs from that already described. 



It may be noted at the start that no marked exceptions were found 

 to the previously described condition, although at least two stages in 

 the process of abscission have been found to be subject to considerable 

 variation. The first of these stages has to do with the place of origin 

 of the abscission process itself. An independent origin in the pith 

 has been demonstrated to occur in a large number of species and 

 occasionally it was found that the first evidences of abscission could 

 be detected here before any similar evidences appeared in the cortex. 

 Again, it was found in most species that cell separation starts first in 

 the ventral cortex although other places of origin were found in 

 several eases. Thus, in Xirotiana Tabacum "Maryland" and F, H36, 



