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 steve 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 
steve-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 
