30 THE FRUIT OF OPUNTIA FULGIDA. 



end of the season of its origin the fruit is protected by the well-cutinized 

 epidermis and the underlying hypodermis, except for the perianth-scar and 

 the individual leaf-scars. This epidermis persists over the general surface 

 of the fruit for several years. Only where the epidermis is cracked by the 

 swelling of the fiTiit or is otherwise injured is the original protective layer 

 replaced in function by cork, like that of the perianth-scar. 



The periderm of the perianth-scar has its origin, as we have noted, from 

 a pliellogen layer arising in descendants of the original cells of the abscission 

 layer, just within the plane of abscission (figs, 62, 63). The first 5 or 6 

 layers of cork cells formed by the cork cambium are very thin-walled ; then 

 there is formed a single layer of very thick cells (see Wolfe, 1912, figs. 6, 

 7, 9). These sclerenchyma-like cells have walls of a pale-yellow color, 

 which in older fruits are made up of from 15 to 20 distinct layers marked by 

 numerous minute radial pits (figs. 63, 64, 65). The walls stain intensely 

 with safranin and gentian violet. The cork cambium, after forming this 

 thick layer, may continue to form more thin-walled cork cells until, in the 

 older fruits, 12 to 15 or more layers are present inside the intact, thick layer. 

 These later layers are more numerous on the upper margin of the perianth 

 scar. With increasing age the 5 or 6 layers of thin cork outside the scleren- 

 chyma layer, which are left more or less shriveled after abscission, are grad- 

 ually worn off from the more exposed parts of the scar. This leaves the 

 sclerenchyma as the superficial and essentially protective layer unless this is 

 injured. When this primary sclerenchyma layer is broken a secondary one 

 is formed immediately outside the then active phellogen (fig. 14, at right). 



Periderm formation in the region of the areole begins soon after the fall 

 of the leaf, in an area that includes tissue of the leaf -scar itself and a few 

 of the epidermal cells below and beside this (fig. 14). There is thus formed 

 a periderm of from 3 or 4 to 8 or 10 cells in thickness, of which the portion 

 immediately above the foliar bundle may finally include 3 or even 4 immedi- 

 ately superposed sclerenchyma layers (figs. 63, 64). As development of the 

 areole proceeds the cork layer develops upward from the leaf-scar and 

 thin-walled cork-like but apparently little-suberized cells are formed imme- 

 diately beneath the persistent trichomes, just above the leaf. Later the 

 formation of these cells continues on beneath the bases of the withered nec- 

 taries and spines if there are any, and finally beneath the bristles (figs. 14, 

 17). The latter, in older fruits, may be supported upon 12 to 15 or more 

 layers of clear thin-walled cork-like cells (fig. 14). The only parts of the 

 areoles that are not covered are the growing-point and the immediately sur- 

 rounding series of rudiments of nectaries, bristles, etc. These all lie well- 

 protected at the base of the dense tuft of younger tricbomes that overhang 

 the growing-point (fig. 50). 



On the lateral surface of the older fruit corky tissue may arise by the 

 extension of the cork about the areole, or that of the perianth-scar over the 

 edge of the cup,' or it may arise de novo from injured spots in the clear epi- 



