80 



In other cases the cambium forms much more wood or bast 

 at certain points than at others, and, on a cross section., radial 

 masses of wood and bast are formed alternating with each other, 

 the cambium forming an undulating layer instead of a circle. 

 Periderm 76. In those Dicotyledons and 



Gymnosperms in which the interior of the stem is con- 

 tinually increasing in thickness through the activity of the 

 cambium ring, the fundamental tissue and epidermis, which 

 at first surrounded the primary vascular bundles, is sub- 

 jected to pressure and tension. In some cases the cells com- 

 posing these tissues continue to grow and divide and thus 

 keeping pace with the internal development are able to* 

 persist. This occurs in most annual stems and in perennial 

 stems during the first year of their development and occa- 

 sionally for a longer period. In the majority of perennial 

 stems, to wards the close of the first year's growth, the epidermis 

 being no longer able to withstand the strain becomes ruptured 

 and falls off. Before this happens, however, a layer of cells 

 has been produced close below the epidermis which acts like a 

 secondary cambium and is called the Phellogen 9 or Cork Cambium. 

 By the active growth and division of this layer new cells are 

 added both towards the exterior and interior of the stem. The 

 former lose their protoplasm and become converted into cork, 

 forming the so-called Phellem., while the latter retain their 

 protoplasm and usually contain chlorophyll, constituting the 

 Phelloderm. The term Fender m includes the Phellem, Phellogen 

 and Phelloderm. The cork cells of the phellem form an elastic 

 protective covering to the stem which is almost impermeable 

 to air and water and which effectually takes the place of the 

 ruptured epidermis. It is owing to the formation of this layer 

 of cork that the colour of young twigs usually changes from 

 green to some shade of brown, the chlorophyll in the cells of 

 the fundamental tissue beneath it being no longer visible as 

 was the case when these cells were covered by the thin epider- 

 mis. In some cases the periderm, instead of originating close 

 below the epidermis, is formed deeper down in the funda- 

 mental tissue, and sometimes close to the vascular bundles. In 

 such cases, a? all the living cells outside it are prevented from 

 obtaining the supply of water and food necessary for their 

 existence, on account of the impermeable cork layer, they diy 

 np and die, forming the rough outer bark which is sooner 

 or later exfoliated. In some trees, e.g. the Birch, the first 

 formed, or primary, phellogen remains active for several years 

 and in some species it persists through the entire life of the 

 tree, new cork cells being added below as the outer ones are- 



