426 

 Cork Outgrowths. 



Cork is universally formed as normal tissue. It may increase abnorm- 

 ally, forming an excrescence under special circumstances. Even the regular 

 formation of cork may be observed in varying amounts in different seasons. 

 Attention should be given to the usual bark pores with their rounded com- 

 plementary cork cells separated by intercellular spaces. These cells, show- 

 ing for some time a cellulose reaction, are steadily reproduced during the 

 time of growth. In winter, when the exchange of gases in the dormant 

 bark is at its minimum, the production of the complementary tissue is 

 stopped. In the autumn a layer of normal cork is formed from the cambium 

 layer instead of the roundish complementary cork cells. With the awaken- 

 ing of bark activity in the spring, the cork cambium again forms comple- 

 mentary cork, rupturing the winter covering layer of the lenticel, just as, 

 when the first lenticels were formed, it had split the epidermis. The more 

 moist the air becomes, the more frequently the elongating complementary 

 cork cells which attract water are formed on the surface of the bark. The 

 longish, mealy, white excrescences, which may be rubbed off, are well- 

 known. They occur on the smooth barked trunks of cherries and alders 

 in damp habitats when the atmospheric humidity is increased and the foliar 

 transpiration decreases. 



At the base of the strong petioles of Juglans regia, Sambucus nigra, 

 Ailanthus glandulosa, Pauloivna imperialis and other trees, in the autumn, 

 structures may be observed very similar to lenticels, only the cambium layer 

 is missing (Stahl)^ Later research- has shown that cork cushions not only 

 develop at the base of the petiole but, in many plants, at the veins of the 

 under side of the leaf (Ficiis siipulata) and finally also on the leaf-blades. 



Now, although this formation of cork on the leaf-blade is a phenom- 

 enon almost as widely distributed as that on the stems with which it closely 

 corresponds in structure and development, yet, in spite of the wide distribu- 

 tion, there is no pathological significance in these formations. 



In these cork outgrowths of leaves, two types may be distinguished'', 

 either the cork layer with its dividing walls and its usually one-layered 

 phellogen lies parallel with the leaf surface in the same plane, — when the 

 cork excrescences are raised above the surface of the leaf in the form of 

 warts ; or the cork layer and especially its phellogen lies in the form of hour- 

 glass-like, depressed zones in the interior of the leaf and usually becomes 

 deeper and deeper. Many plants have both forms on the same leaf. In 

 contrast to the regularity of the appearance and production of stem cork, 

 emphasis should be placed on the accidental appearance of cork excrescence 

 on leaves. Aside from the fact that the two above mentioned types can 

 begin on the same leaf, there are also transitions between the two types. In 



1 Stahl, Entwicklungsgeschichte und Anatomie dcr Lenticellen. Bot. Zeit. 

 1873, No. 36. 



2 Poulsen, Om Korkdannelse paa Blade. Kjobenhavn 1S75. 



3 Bachmann, tlber Korkwucherungen auf Blattern. Pringsheim's Jahrb. 1880, 

 Vol. XII, Part 2, p. 191. 



