266 
STEMS 
for growth, and (2) it does not result in the loss of any 
wood once formed. On the other hand, the plant which 
makes an indefinite annual growth can take advantage of 
favorable growing conditions whenever 
they occur, although it may lose some of 
its youngest wood if frost comes before it 
is hard enough to resist it. 
In the annual growth of woody stems, a 
new node is added to each branch, and a 
new. layer of wood over the wood of previ¬ 
ous years. This makes it possible to tell 
approximately how many years old a tree 
is, the growth of each year forming a ring 
more or less distinct according to condi¬ 
tions. When the conditions for growth 
are at their best, the cells formed are large. 
As conditions become less favorable, the 
cells become smaller and have thicker 
walls, marking distinctly the end of one 
season’s growth from the beginning of the 
next. The size of the cells varies greatly 
in different trees, producing the “grains” 
which are so characteristic of each kind 
of lumber. 
233. Vascular Bundles or Fibro-vascular 
Bundles. — Vascular bundles are a part of 
the conductive system of the plant. They 
are in stem, root, and leaves. In the 
leaves we call them veins. In celery they 
are the “ strings ” (see Figure 256). 
234. Structure of a Vascular Bundle.—Avascular bundle 
in a monocotyledonous plant is made up of two kinds of cells 
or groups of cells called xylern (zy'lem) and phloem (flo'em). 
The xylem cells are thick-walled, but with thin spots in each 
cell where it touches another cell of the same kind. Each 
Figure 254. — Wood 
of Spruce, Greatly 
Magnified. 
A, large cells 
formed early in sea¬ 
son ; B, E, small 
cells formed late in 
season; C, resin 
duct; D, medullary 
ray. 
