SECT. I MORPHOLOGY 157 



epidermal cells, and at the same spots similarly thickened cortical 

 cells are developed. The epidermal cells thus cut off die, and 

 the original epidermis is more or less completely replaced on such 

 old regions of the stem. In the great majority of plants in which 

 the epidermal layer persists for years it is able to meet the demands 

 made upon it by the growth in thickness of the stem without the 

 assistance of the underlying cortex. The stems of one of the Maples 

 {Acer striatum), even when a foot or more in thickness and over forty 

 years old, remain covered with a living, growing, epidermal layer. 

 The striations to which the specific name of this plant refers are 

 caused by a white wax exuding from splits in the outer walls. As a 

 rule, however, the epidermis on stems which grow in thickness 

 becomes stretched and finally ruptured. The tissue of the primary 

 cortex and of the bast by the expansion and division of its cells, can 

 accommodate itself more easily than the epidermis to the increased 

 dimensions of the stem, arising from the growth in thickness of 

 the central cylinder. This process of accommodation is particularly 

 noticeable in the primary medullary rays (Fig. 155 pni) between the 

 primary phloem. The formation of the peridj:rm generally begins 

 during the first vegetative period, after the secondary growth has 

 reached a certain stage. The commencement of its formation is 

 indicated by the brown colour of the external surface of the stem, 

 which remains green so long as the epidermis continues alive. The 

 formation of periderm usually occurs at a later period in those plants 

 which have a persisting epidermis ; it is practically wanting only in 

 the species of Mistletoe. The periderm is derived from a secondary 

 meristem, termed the CORK CAMBIUM or phellogen. This phellogen 

 may arise, in the epidermis, in a deeper layer of cells of the primary 

 cortex, or even in the pericycle itself. The phellogen Q^^) is as a 

 rule a cambium with a persisting initial layer from which cork-cells 

 are produced to the outside and phelloderm to the inside. 



The production of cork-cells is usually greater than that of phelloderm. If the 

 phellogen forms only cork-cells the initial cells are often used up in the process, 

 and are then replaced from the adjoining ground tissue (p. 139). This is found 

 among Monocotyledons and also in some Dicotyledons, e.g. Valeriana officinalis. 

 The tissue formed by the phellogen to the outer side is termed phellem by 

 V. HoHNEL. Usually it consists only of cork-cells, but in certain cases unsuberised 

 layers of cells are formed. These are the phelloid cells of v. Hohnel. 



The young cork-cells usually have a tabular shape, fit closely 

 together without intercellular spaces, and possess suberised, secondary, 

 thickening layers. The cork cells usually possess brown walls. 

 They may be thin (Fig. 164/) or thick, frequently thickened on one 

 side (Fig. 166), and occasionally to such an extent that they are 

 known as stone cork. Thin-walled cork-cells usually contain air; 

 those with thickened walL frequently have reddish -brown cell 



