172 THE GROWTH OF ROOTS AND STEMS. 



the epidermis may be replaced by cork, which serves chiefly 

 for protection, but allows the passage of gases through the 

 lenticels, which have the same function as stomates. 



The ground-tissue, which is continuous throughout the 

 plant, consists primarily of living thin- walled cells, separated 

 by air-spaces which form a continuous system and allow 

 gases to circulate through the plant. In the root the 

 ground-tissue consists of colourless cells ; its chief functions 

 are to convey water from the root-hairs to the wood- vessels, 

 to carry oxygen to the living and growing cells of the root, 

 and to store up food. In the stem, the outer part of the 

 ground-tissue consists of chlorophyll-containing cells which 

 carry on photosynthesis, especially in plants whose leaves 

 are very small or do not remain long on the plant e.g. 

 Horsetails, Grorse, Broom, Asparagus, Bilberry. Part of 

 the ground-tissue of the stem is usually modified to form 

 strengthening tissue (collenchyma, sclerenchyma) . 



The ground-tissue of the leaf is simply an extension of the 

 green outer tissue of the stem, the cells having larger air- 

 spaces between them, and being specially adapted to photo- 

 synthesis, while the epidermis contains more stomates. 



The vascular tissue (wood and bast) is also continuous 

 throughout the plant. In the root the wood and bast are 

 in separate bundles, but in the transition region between 

 root and stem the root-bundles change in position so that 

 the wood and bast form double or " conjoint " bundles, the 

 wood coming to occupy the inner side of each stem bundle, 

 twisting round so that the first-formed vessels (spiral and 

 ringed) are nearest the centre in the stem, instead of being 

 external as in the root. The essential parts of the bundles 

 (vessels and sieve tubes) run continuously from root to 

 leaves, the stem-bundles bending outwards into each leaf so 

 that the wood is on the upper side of each leaf -bundle (vein) 

 and the bast on its lower side. 



We have therefore three systems of conducting channels 

 running through the plant: (1) the wood- vessels which 

 carry water and dissolved salts ; (2) the bast-tubes (sieve- 

 tubes) which carry manufactured organic food, especially 

 nitrogenous substances ; (3) the air- passages between the 

 cells of the ground-tissue, through which air circulates, the 

 exchanges of gases between the atmosphere and the living 



