f CORTEX tts 



they are especially rich in cell-sap, which is often tinted pink, 

 red or purple by a colouring matter which appears to protect the 

 cells of the cortex from excessive light. In some plants, if not 

 in all, the cell-sap of the epidermal cells functions as a store 

 of reserve water upon which the more internal cells of the stem 

 can draw in time of need. 



It is well known that the surface of stems and other parts of 

 plants are frequently covered with hairs. These belong to the 

 epidermis, and in their simplest form are merely single cells 

 which have grown much longer than their neighbours. Some 

 hairs are, however, multicellular extensions of the epidermis 

 (h, Fig. 54), and like the unicellular hairs may assume a great 

 variety of shapes. 



Hairs are often harsh to the touch, and furnish a means of 

 defence against insects and animals generally. They also act as 

 a mantle which prevents too rapid escape of water from the 

 plant, and acts as a screen against excessively bright sunshine. 



In young stems and buds, hairs protect the tender parts 

 against injury by frost. Certain hairs function as secreting 

 organs, and are then designated glands (Fig. 106) : they often 

 produce resinous and oily compounds, which in the case of 

 mint, hop, and other plants have a characteristic odour. Many 

 excreted products of such hairs are sticky, and effectually prevent 

 insects such as ants from climbing up the stem and getting 

 at the nectar of the flower. 



(ii) The cortex of the stem extends from the epidermis to 

 the vascular cylinder. A great part of it generally consists of 

 living parenchymatous cells which contain abundant chloroplasts. 

 The cells of the portion immediately beneath the epidermis 

 frequently have their cell-walls thickened at the corners, and 

 form what is spoken of as collenchymatous tissue : the latter serves 

 to strengthen the epidermis, and gives rigidity to the whole 

 stem. The innermost layer of cells belonging to the cortex 

 forms a continuous sheath surrounding the vascular cylinder 



