56 COMPENDIUM OF GENERAL BOTANY. 



The thin walls have, so to speak, become sacrificed to the first two 

 functions (p. 53). 



This explains the water-storing and the mechanical function of 

 the tegumentary system. We must now discuss somewhat more in 

 detail the function of " protection against evaporation." For a long 

 time we have made a practical and technical use of the peculiar 

 properties of corky or cuticularized membranes. They are used 

 in a similar manner as by plants. Cork serves to close vessels con- 

 taining liquids, to prevent leakage. Sometimes sealing-wax, resin, 

 or some other waxlike substance is added to prevent excessive loss 

 by evaporation. So we find waxy excretions and waxy coatings on 

 plants of those climates with periods of dryness. YOLKENS* reports 

 a desert-plant whose leaves are coated with a resinous substance. 

 This structural change corresponds with a functional increase of the 

 cuticula. 



We now come to the consideration of cork and bark. These 

 formations, though they may form layers several inches in thickness, 

 are nevertheless physiologically related to the cuticle, which is 

 frequently immeasurably thin. 



Cork and Bark. While, as above stated, the epidermis consists 

 of a single layer of cells, the bark-covering, as a rule, consists of 

 several or many layers of cells. Cork several layers in thickness 

 may result from simple cnticularization of ordinary parenchyma- 

 cells, but in the majority of cases cork is the result of a special 

 process of cell-division. This process of cell-division has the 

 greatest similarity to " cambial activity," that is, to the cell-form- 

 ing process in the ring between the wood and bark (cambium -ring) 

 of our trees. 



In the case first mentioned the cork-cells do not necessarily lie 

 in radial series, while in the latter case this arrangement is charac- 

 teristic. The cork-cambium (phellogen), as well as the above- 

 mentioned cambium between wood and bark, is, as a rule, a bipolar 

 formative tissue. In only a few cases it is one-sided, that is, forms 

 cells which become cuticularized from without inward. Ordinarily 

 in bipolar cork-cambium activity the numerous outer cells become 

 cuticularized centripetally. There are formed inwardly less numer- 



1 The same investigator observed a shrub (Reaumuria Mrtella) in the Arabian 

 desert in which epidermal glands secrete a hygroscopic saline substance which 

 absorbs moisture from the air during the night. 



