156 PLANT ORGANS OR PARTS OF PLANTS. 



regions of growth, the periblem, the plerome, and the 

 dermatogen, from which within general Hmits the ground 

 tissue, the fibro-vascular bundles, and the epidermis 

 severally take their origin. 



Primary epidermis is found in few official barks and is 

 to be seen best in some of the official herbs. In young 

 stems this layer is generally one or two cells thick, the 

 outer cells being cutinized. 



Secondary epidermal structures are common. Among 

 the official plants are found the following types. 



In the stems of most dicotyledons and a few monocot- 

 yledons a series of changes take place which make up the 

 secondary epidermal structures. These secondary changes 

 are brought about by means of what is known as the 

 phellogen layer. As the stem of a plant gets larger the' 

 primary epidermis is forced off, and if no other structures 

 grew, the tissues of the plant would suffer exposure; 

 so that while the increase in size is taking place and the 

 primary epidermis is being thrown ofif a layer of cells 

 immediately beneath the primary epidermis takes on 

 meristematic growth, forming the phellogen layer, and 

 builds up the periderm, adding new structures mainly 

 composed of isodiametric cells on both inside and out- 

 side. 



The modifications of these isodiametric cells on both 

 inside and outside make up a number of distinct types, 

 the most important of which are the cork cells. These are 

 generally the outermost. Beneath these parenchymatic 

 cells, which may be thickened, stone cells may be found. 

 Within the phloem elements are present. 



Hence from the pharmacognostic standpoint the follow- 

 ing structures are to be identified and studied : Cork cells ; 

 stone cells; bast fibres; sieve tubes, generally lost or 

 dried or otherwise mutilated; simple parenchyma. 



