STEMS 53 



have numerous thin spots in their walls which look like 

 dots of various sizes, and these are the dotted or pitted 

 vessels (Fig. 53, C), often called dotted ducts. These pitted 

 vessels are often very large, their openings being visible to 

 the naked eye in the cross-section of oak wood. 



The cells of the bast that conduct prepared food are 

 called sieve vessels (Fig. 53, D), because in their walls, 

 usually the end walls, there appear areas full of perfora- 

 tions, like the lid of a pepper-box, these areas being called 

 sieve-plates (Fig. 53, E). 



The veins of leaves are vascular bundles that are 

 continuous with those of the stem. If the relative positions 

 of wood and bast in the stem be remembered, it will be 

 seen that when a bundle turns out into a leaf, the wood 

 with its tracheary vessels is toward the upper side of the 

 leaf, and the bast with its sieve vessels toward the lower 

 side. 



A prominent feature of such stems is that they can 

 increase in diameter. If the stem lasts only one growing 

 season, that is, if it is an annual, the increase in diameter 

 does not occur; but if it lasts through several seasons, that 

 is, if it is a perennial, it increases in diameter from year to 

 year. Naturally annual stems belong to herbs and perennial 

 stems to shrubs and trees. Taking the tree as an illustra- 

 tion, the increase in diameter occurs as follows: Between 

 the wood and the bast of each bundle is a layer of very 

 active cells called the cambium (Fig. 52, c), which soon 

 extends across the intervening pith rays, and so forms a 

 complete cylinder of cambium. This cambium has the 

 power of adding new wood cells to the outer surface of the 

 wood, and new bast cells to the inner surface of the bast, 

 as well as adding to the pith rays where it traverses them. 

 In this way a new layer of wood is laid down on the outside 

 of the old wood; and usually these layers, added year after 

 year, are so distinct that a section of wood shows a series 



