14 THE STORY OF PLANT LIFE 



salts, and of their storage. Certain tissues, the 

 corky tissues, serve as a waterproof covering, and for 

 protection. The bast fibres, etc., serve as girders or 

 strengthening supports. Climbing plants have, 

 hov^ever, to rely upon the support of others. The 

 thin epidermis itself is a means of protection, being 

 often covered with hairs, spines, etc. It also serves 

 to prevent the loss of water, and protects the internal 

 cells from mechanical injury. Superficial glands 

 serve to get rid of, or to store up, oils and resins, etc. 

 The cells are not green, but bright red or coloured, 

 to protect the cortex from too intense light. In the 

 wood the long tubes usually contain no living proto- 

 plasm, but serve only for water conduction or storage. 

 The same applies to the branches, which form parts 

 of the same circulatory system for water and food- 

 materials. 



The leaves have essentially the same kinds of 

 tissue as the stem, but their arrangement and 

 structure is not the same. They contain bundle 

 sheaths which transfer carbohydrates from the leaves 

 to the stem. The leaves are thin, usually broad, 

 and expose a greater surface, relatively to their 

 volume, than other structures. They play, in fact, 

 the important part of presenting the chloroplasts to 

 the action of the light. 



The mesophyll of the leaf consists of long vertical 

 palisade cells above in which the chloroplasts mainly 

 occur. 



Below is the spongy parenchyma, in which are 



