234 BARKS 



Tegumentary Tissue. Few barks are so young when collected as 

 to retain their epidermis ; this tissue has usually been replaced 

 by a cork of varying thickness. Sometimes the development of 

 successive layers of phellogen deeper and deeper in the cortex, and 

 possibly in the secondary bast, has resulted in the formation of cor- 

 responding layers of cork, the tissues thus cut off losing their vitality. 

 To the mass of protective tissue (the * bark ' of botanists) thus formed 

 the name of * outer bark ' is commonly assigned by pharmacognosists. 



The characters of the cork, influenced in some cases by the nature 

 of the cell contents (cascarilla, alder buckthorn), often afford a valu- 

 able means of identifying a bark. 



Phelloderm is seldom formed in quantity sufficient to be diagnos- 

 tically important. Canella bark alone contains a sclerenchymatous 

 phelloderm easily visible under a lens. 



Cortex. This tissue, often termed ' primary,' or ' middle ' bark 

 by pharmacognosists, is the tissue extending from the epidermis, 

 cork, or phelloderm, as the case may be, up to and including the 

 endodermis. Beyond the presence or absence of groups of scleren- 

 chymatous cells, of oil-cells, oil-glands, or calcium oxalate, it seldom 

 affords any well-marked characters. 



Bast. The primary bast being seldom distinguishable even under 

 a microscope, this tissue is practically composed of secondary bast. 

 It comprises the tissue extending from the endodermis to the cambium, 

 and corresponds to the inner bark of many pharmacognosists. Its 

 structure should be carefully examined, as it is frequently of great 

 diagnostic importance. 



It occasionally happens that the cells of the pericycle thicken 

 and lignify, and thus form a band of sclerenchymatous tissue, readily 

 visible under a lens, in the mature bark (sassy bark, nux vomica 

 bark) ; otherwise the extent of the secondary bast is not readily 

 fixed ; it continues at least as far as the medullary rays extend. 



The chief feature of importance in the bast, and visible under 

 a lens, are the presence or absence of : (i) sclerenchymatous 

 cells ; (ii) bast fibres ; (iii) oil-cells or glands ; (iv) calcium oxalate ; 

 (v) mucilage. 



Sclerenchymatous cells commonly occur in rounded, less often in 

 tangentially elongated, groups exhibiting a uniform semi- translucent 

 appearance. They impart hardness to the bark and render the 

 fracture short and granular (sassy bark) . 



Bast fibres may occur scattered or in groups, which are then often 

 tangentially elongated and arranged in tangential lines. They im- 

 part to the bark a tough and fibrous character ; from the fractured 

 surface of such a bark the fibres usually project (oak, elm, &c.). 



If the bast fibres approach sclerenchymatous cells in nature the 

 the fracture becomes splintery (quillaja). 



Oil-glands or oil-cells usually appear as minute yellowish, reddish 



