26 BOTANY 



development of the wood on these lines was the mechani- 

 cal advantage which was thereby gained, for, with the 

 same number of thick-walled wood cells, a stronger 

 column is produced when it is in the form of a cylinder 

 than when it is solid. The wood cells in the stem have 

 not only to conduct the water current to the leaves, 



FIG. 3. Transverse section of part of a stem of Aristolochia, showing the 

 different kinds of ground tissue and vascular cells. The four largest cells 

 in the centre are wood vessels, and the narrow layer of cells just behind 

 them, is the cambium layer which gives rise to the new tissue year by year. 



but have also to play a large part in making the stem 

 strong enough to stand upright. 



As the stem gets older the ring of secondary wood 

 and bast increases greatly, and in perennial plants solid 

 rings of wood are added year by year which soon dwarf 

 the original primary groups of wood, and they cease 

 to function after a time. In trees and woody shrubs 

 the formation of the secondary zones of wood increases 

 largely, and they become the principal feature in the 

 trunk. 



