8/8 A TEXTBOOK OF THEORETICAL BOTANY 



stomatal openings. The phellogen formed at these spots is highly active and 

 cork formation is in progress there before the phellogen has spread round 

 the stem as a continuous layer. The excess of cork cells formed, ruptures 

 the epidermis around the stoma, and protrudes above the general surface 

 level as a spongy mass called the complementary tissue. Such a spot is 

 clearly visible to the eye on the stem surface and is called a lenticel. Blockage 

 of diffusion through the lenticels, due to the formation of a closing layer of 

 compact phellem in the autumn, reacts upon the phellogen and stimulates 

 increased divisions, thus bursting through the block in the spring. The life 

 of a lenticel seems to be an alternation of blocking and bursting phases. 

 Permanent lenticels are not in all cases formed at every stoma, the number 

 being related to the vigour of the shoot. In deep-seated phellogens, both 

 primary and secondary, the lenticels are obviously not related to stomata 

 and their distribution appears to be related rather to the broad medullary 

 rays which are associated with the leaf traces. 



Lenticels have been interpreted as ventilating apertures, but their 

 causation has nothing to do with the assumed necessity for this function. 

 Suberin is, however, exceedingly impermeable to air, and the rounded and 

 irregularly arranged cells at the lenticel undoubtedly do allow access to the 

 underlying tissues for air, which cannot penetrate at any other point. The 

 physiological value of this is reflected in the bad effects on the health of the 

 plant which result if the lenticels become covered with external growths of 

 moss or lichen. 



While the chief activity of the phellogen is in forming phellem outwards, 

 there is usually also a slow production of cells inwardly, which are added to 

 the cortex and are known as phelloderm or secondary cortex. These cells 

 are not suberized and are only distinguishable from the cortical parenchyma 

 by their arrangement in radial files and sometimes by the presence of starch 

 grains. Their walls are often thicker than those of the cells of the primary 

 cortex and may even be lignified, as in Canella. 



We thus see the elaboration of a secondary protective system consisting 

 of phellem, phellogen and phelloderm which goes by the collective name of 

 periderm. This slowly develops into the bark of old stems, but in this 

 development other tissues become involved. 



In smooth-barked trees the first phellogen remains active for some years 

 and is only slowly replaced, if at all. The bark of such trees therefore consists 

 of a continuous cork film (Fig. 865). In rough-barked trees, however, the 

 first phellogen is soon replaced by a series of others, lying successively deeper 

 in the tissues, and all later phellogens, after the first few, are formed in the 

 secondary phloem, much of which is destroyed in this way and its remains 

 incorporated in the bark, along with dead cortex, collenchyma, sclerenchyma, 

 etc. Bark is thus a composite structure. Where the later phellogens form 

 continuous zones the bark may show successive annual rings, like the annual 

 rings of the wood, each marking the formation of a new phellogen, but in 

 many other cases the later phellogens form a series of discontinuous arcs 

 and the bark is thus cut up into distinct areas, between which deep cracks 



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