215 



middle of summer. These are filled full of starch, while the normally 

 constructed wood, excepting the medullary rays, has none. Within these 

 cross bands the medullary rays are broadened and have gummy spots. 



The beginnings of the tan formation are found close under the terminal 

 buds of the topmost branches, where the epidermis is still uninjured, but is 

 already underlaid with cork, possibly five layers thick. In places, this pro- 

 tective layer, consisting of comparatively thick walled cells, corresponding 

 to plate-cork, shows a change even in its first stages, so that the cells lying 

 directly beneath the epidermis have developed into parallel rows of cylindri- 

 cal, radially elongated; brovvm-walled, full-cork cells. There is present here, 

 therefore, the character of lenticel growth which Stahl^ has already de- 

 scribed thoroughly for the cherr}' and which only differs from his descrip- 

 tion in that here the full cork cushions are rarely produced under the 

 stomata. 



It is seen that an extensive formation of full cork can take place in- 

 dependently of the stomata in the development of a plate-cork layer, since 

 several layers of lenticels are produced in which the cork formation ad- 

 vances inward into the primary and, in fact, into the secondary bark. 



As the shoot of the current year becomes older, a second layer of plate- 

 cork appears very normally, directly beneath the one first produced. It has 

 been found just as thick (viz., 5 to 7 cells) as the first whose cells gradually 

 collapse with the apparently lessened swelling and the browning of the walls. 

 During this process the normal cork covering of the cherry trunk appears 

 to be differentiated into two layers. The upper, older one is very dense, 

 since the cells usually have so collapsed that their cavities are recognizable 

 only as fine fines ; this layer passes over gradually into the second, later 

 formed cork layer. In the latter, the plate-like cells are very uniform and 

 their wide lumina are filled with a watery content or even with air. They 

 border on a browned cell layer, with a clearly protoplasmic wall lining, 

 which, as cork cambium, assumes the continued formation of the cork layer 

 occurring in places. When treated with sulfuric acid, the composition of 

 the oldest, sunken, collapsed brown cork layer is easily recognizable, since 

 the cells are often distended and show in places their original height and 

 width, at times almost square in cross-section, while the full cork cells are 

 not changed. With this treatment the layer, produced later, rounds out its 

 youngest cork cells into hemispheres after the cork cambium has been 

 destroyed. 



In the formation of the many layers of lenticels, the development of 

 such elements is repeated in the secondary cork layer underneath the first 

 centres of full cork. 



The second case of lenticel formation, not connected with stomata, is 

 illustrated in figure 28. This shows the cross-section of a new structure on 



1 Stahl, Entwicklungsgeschichte und Anatomie der Lenticelle. Bot. Z. 1873, 

 No. 36. 



