374 University of California Publications in Botany [ V L - 5 



noticed by agriculturists is of this type. Quite often, however, true 

 abscission and this second type of flower-fall may both be found 

 operative in the same plant or even in the same flower. "Spon- 

 taneous" flower-fall in the tomato is, of course, of the true abscission 

 type. 



Corresponding with the condition in Nicotiana, true abscission in 

 Lycopersicum is seen to originate frequently in the pith. At any rate, 

 the process goes on here independently of that in the cortex, since the 

 final break is through the tracheae and epidermis. Furthermore, separ- 

 ation takes place in a plane with the bottom of the groove (pi. 53, fig. 

 2) whereas, in Nicotiana, it takes place a short distance distal to the 

 groove. Separation may at first take place between only two tiers of 

 cells (pi. 53, fig. 2), but in time the process may spread until three 

 or four tiers become involved in separation. However, there is no 

 spreading of the process to a large number of cells, as is frequently 

 seen in Nicotiana, so that one very seldom finds the white powdery 

 substance at the point of separation. Also in contrast with the con- 

 dition in Nicotiana, there is, as abscission progresses, no bulging of 

 the epidermis which instead soon breaks in the bottom of the groove. 

 Separation in the tomato takes place in such a way as to give the 

 exposed separation surfaces the same general shape after abscission as 

 in Nicotiana, that of the detached portion of the pedicel being convex 

 and that of the remaining portion slightly concave. 



c. DATURA 



Conditions in Datura differ strikingly from those in the two 

 species described above. This would be expected when one considers 

 the great differences in the structure of the separation zones (cf. 

 page 365). In Datura there is the usual chlorophyl-bearing tissue, 

 which consists of two rows of small, perfectly isodiametric cells with 

 large intercellular spaces, just beneath the epidermis. It will be 

 remembered from the description on page 365 that this tissue in Datura 

 continues the entire length of the pedicel and therefore, in contrast 

 with the condition in Nicotiana and Lycopersicum, extends through 

 the separation zone. The first sign of abscission is the maceration of 

 this tissue as indicated by the appearance of a white color under the 

 epidermis. The latter may as a result become detached from the tissues 

 of the cortex for a distance of 2 cm. or more along the base of the 

 pedicel. This is soon followed by a break over the separation layer 

 and a curling back of the epidermis on either side, with most of the 

 chlorophyl-bearing cortical tissues still attached to its inner surface. 



