544 THE STRUCTURE OF ECONOMIC PLANTS 



mately is supplied by one large median and four lateral bundles. 

 Above the point of departure of the traces to the petals, five 

 staminal traces diverge that alternate with those of the petals 

 and are opposite the main traces of the sepals. 



Above the level at which the stamen traces diverge, the vascular 

 system is more or less elliptical in transection; and, at each end 

 of the ellipse, a small bundle passes outward and upward, one 

 opposite a petal and the other opposite a stamen. These are the 

 abaxial carpellary bundles which extend into the style for a 

 distance nearly equal to its length. The main vascular tissue 

 again becomes continuous, closing the gaps so that the stele 

 appears in transection as two crescentic masses broken by traces 

 extending outward and upward through the lateral walls of the 

 carpels. At a higher level, the vascular cylinder is divided into 

 many small bundles, some anastomosing to form a vascular net- 

 work while others extend into the ovules. Thus the vascular 

 supply to the ovules is apparently cauline rather than foliar. 



Floral Abscission. — Some varieties habitually produce more 

 flowers than others; but in any given variety, flower production 

 may be extremely variable from year to year. The formation of 

 flowers does not necessarily mean that development will continue 

 to the point where the fruits or "seed balls" are produced con- 

 taining viable seed, and it commonly happens that abscission of 

 the flower may occur at some stage prior to the maturation of the 

 fruit and seed. Dorsey (lo) has investigated this matter, and 

 reports that the abscission of the flowers bears no essential relation 

 to abortion of pollen or failure to receive a stimulation from polli- 

 nation and fertilization. He regards abscission as being due to 

 "physiological influences operating independently of pollen or 

 pistil development." 



Young (i9) has described the shedding of the blossoms in some 

 detail and points out that the abscission occurs at a definite point 

 about half an inch below the base of the blossom where there is a 

 joint in the pedicel of the flower. It is at this point that the 

 abscission layer is formed. This is marked at a very early stage 

 by a constriction in the pedicel beneath which the cells are smaller 

 than in the tissues immediately above or below. These cells 

 have dense protoplasmic contents with large nuclei, and form 

 a meristematic zone that is capable of growth and cell division 

 after the adjacent cells have become inactive. The abscission 



