232 THE CARBOHYDRATES 



esters and alcohols. Legg and Wheeler, after saponification 

 of Agave cuticle with alcoholic potassium hydroxide, isolated 

 cutic acid CgeHgoOg and cutinic acid C13H22O3, which they 

 consider to be the constituents of the acid described by Fremy 

 and Urbain * as oleocutic acid. 



SUBERIZED MEMBRANES. 



In the majority of trees and shrubs and in many her- 

 baceous plants, the superficial tissue, or tissues, is replaced 

 by a secondary tegumentary system. This normally arises 

 after the primary tissues are fully differentiated and, generally, 

 soon after the beginning of secondary thickening in the vas- 

 cular system. This secondary tegument is known as periderm, 

 and its formation is instituted by the advent of a new meri- 

 stem, the cork cambium or phellogen. In the stem, the 

 phellogen may arise in the epidermis, as in Nerium ; in the 

 hypodermis, as in Samhucus ; or in the deeper layers of the 

 cortex, as in Ribes. In the root, the phellogen generally has 

 its origin in the pericycle, although in some instances it may 

 arise in the superficial parts of the cortex as in V alerianella. 



The segmentation of the phellogen results in the formation 

 of regular serial rows of closely packed brick-shaped cells 

 towards the exterior and, more especially when the phellogen 

 is deeply seated, a less regular and more or less extensive 

 series of cells towards the interior. The former undergo a 

 gradual change, suberization, lose their living contents, and 

 finally become cork, whilst the latter retain their living 

 contents and form a secondary cortex, known as phelloderm. 

 The formation of cork isolates the tissues on its outer side 

 which thus are cut off from all supplies and die.f Phelloderm 

 thus comprises the dead cork and the dead primary tissues 

 on its outer side, the living phelloderm if formed, and the 

 phellogen situated between the cork and the phelloderm. 

 The cork of commerce is mostly derived from the cork oak, 

 Quercus suher. 



* Fr6my and Urbain : " Ann. Sci. Nat. Bot.," 1882, vi., 360. 

 t The term " bark " often is loosely used. Bark comprises all the 

 dead tissues external to the phellogen. 



