THE ANGIOSPERMAE : STEMS 875 



from one lacuna to another, but they are too small to allow water to penetrate, 

 if it should chance to get into one lacuna through an injury to the epidermis. 

 They are in effect waterproof bulkheads and prevent the accidental flooding 

 of the whole internal air system (Fig. 862). 



4. The Periderm. The epidermis is usually only retained for a short 

 time while the shoot is young, and it is later replaced by a secondary covering. 

 This is necessary because the thick walls and the cuticular layer of the 

 epidermis inhibit the division of its cells. The epidermis can therefore only 



Fig. 862. — Potarnogetoji natatis. Transverse section of stem showing 

 one of the transverse membranes with small intercellular passages. 



accommodate itself to the increasing girth of growing stems by the tangential 

 stretching of its cells, and that is limited in extent. 



A few cases exist of woody plants which retain their epidermis for a number 

 of years, e.g., Lawns, Aucubo, Rosa, Acer. The cuticle is soon ruptured by 

 the growth of the stem, but the cutinized cell wall increases enormously in 

 thickness and is progressively regenerated from within, while the outer layers 

 crack and crumble away. Most smooth-stemmed shrubs do, however, 

 eventually form a bark. One exception is the Alistletoe, in which the primary 

 epidermis is supplemented by further cutinized layers of cells formed 

 successively inwards, so that a multiple epidermis of great thickness is built up. 



Only a growing tissue can retain permanently its equilibrium with other 

 growing tissues, and thus the secondary covering of the stem is initiated 

 by the formation of a cambial layer known as the cork cambium or 

 phellogen, the chief product of which is the phellem or cork. The 



