58 COMPENDIUM OF GENERAL BOTANY. 



are habitutally or accidentally exposed ; on the leaf-scars ; on 

 injured plant-tissues; on subterranean organs which must be pro- 

 tected against excessive moisture (for example, potato-tubers, older 

 roots and rootlets 1 ). In the chapter on Reproduction the cuticu- 

 larization of the outer coverings of pollen-grains and spores 

 will be discussed. This secondary corky change has a bearing on 

 the ability to resist atmospheric changes for a shorter or longer 

 time (resting period). 



In all these cases a protective tissue is required. Usually this 

 tissue has the power of continuous regeneration. We find corky 

 tissues in older roots, in subterranean stems, on leaf-scars, and, most 

 common of all, as a covering of the cambium-ring of growing tree- 

 stems. Each of these cases we must discuss more in detail. 



Scar-tissue ( Wundkork). The prick of a needle into a develop- 

 ing potato-tuber, or into the young stem of a woody plant, causes 

 the death of the injured cells, and perhaps of a few others in their 

 immediate vicinity. In nature such injuries may result from 

 stings or bites of various animals. The uninjured cells surrounding 

 the injured part at once proceed to divide parallel to the injured 

 surface, that is, tangential to the centrally located injured cells. For 

 example, an injury resulting from the puncture made by a needle will 

 develop a cylindrical covering of suberized cells. This scar-tissue 

 separates the injured (in other cases diseased) portion from the normal 

 tissue, and at the same time prevents the evaporation of moisture 

 from the injured surface. 



Falling of Leaves. Before the leaves begin to fall in the autumn 

 a " scission-layer " is formed between the base of the petiole and 

 the stem. A separation of the cells of this layer causes the leaves 

 to fall off. In a large number of instances the formation within 

 the scission-layer of a plate of ice which subsequently melts, 2 

 causes the profuse falling of leaves noticeable in the fall. The 

 scission-layer is, however, not the protective covering. A pro- 

 visional protection is formed by a mucilaginous substance known as 

 callus, which closes the vessels ; or by the ' tyloses," that is, certain 

 cellular protrusions which grow into the vascular system from the 

 cells of its immediate surroundings. Drying forms a provisional 

 protection for the parenchyma (perhaps in connection with a chem- 



1 Also on the root-tubercles of Leguminosw and Cycas revoluta. TRANS. 



2 MOHL, Botanische Zeitung, 1860, and Sachs, Vorlesungen. 



