148 
A TEXTBOOK OF GENERAL BOTANY 
formed in the spring are much larger than those produced in 
the latter part of the growing season (Fig. 130). It is thus easy — 
to distinguish the rings of wood formed in successive years in 
HOQa Lacey X 
© Beoe | 2 
K Aeoness Blan pse6 SA 
ROSSER ges Rasen LOOKS 

Fra. 145. Diagram of secondary 
thickening in a vascular bundle, 
showing four annual rings, la- 
beled 1, 2, 8, and 4, in xylem 
and phloém 
On both sides of the bundle pith 
rays run from the pith, while pro- 
eressively shorter and shorter rays 
are found in the bundle. In the 
phloém region thick-walled scleren- 
chyma alternates with the phloém 
either dicotyledonous (Fig. 142) 
or coniferous trees of the temper- 
ate zones. These rings are called 
annual rings. The width of the 
annual rings varies with the envi- — 
ronmental conditions at the time 
of their formation ; during favora- 
ble seasons wider rings are pro- 
duced than during unfavorable 
seasons. As each ring represents a 
single season’s growth, the age of 
a tree can be determined by count- 
ing the annual rings. Fig. 143 
shows a section of a portion of a 
two-year-old stem with two rings, 
while Fig. 144 represents a por- 
tion of a stem with three narrower — 
rings. Similar rings are sometimes 
produced in tropical countries by 
the alternation of pronounced wet 
and dry seasons; in this case the 
fundamental physiological factors 
involved are essentially similar to. 
those just described. In most trees 
erowing in tropical countries, where 
there are no pronounced changes of 
season, there are no annual rings 
(Fig. 140). 
Secondary pith rays. As the 
course of the secondary thickening 
continues, the outer edges of the xylem and the inner edges of 
the phloém in the bundles naturally become wider and wider. 
They do not increase in width very greatly, however, before 

