146 A TEXTBOOK OF GENERAL BOTANY 
best-known commercial bast fibers are flax (from which linen 
cloth is made), hemp, jute (much used in making coarse sacks), 
and ramie (the so-called China grass). 
Annual rings. In regions with a very pronounced cold season’ 
the diameter growth of woody plants takes place only during 
the spring and summer, and not in the winter. The wood of 
marked off from that 
of the next, because 
the wood formed first 
consists more largely 
of vessels than does 
that formed later, when 
wood fibers are rela- 
tively more abundant 
(Figs. 141, 142). In 
many trees, vessels are 
formed only or largely 
in the first part of the 

production of the ves- 
sels early in the sea- 
son is advantageous 
to trees in several 
Fie. 142. Cross section of a three-year-old 
linden stem 
In the center is the pith surrounded by three an- 
nual rings of wood. In the bark are alternating aie 
strands of phloém and bast fibers respects, and 1S con- 
nected with their sea- 
sonal activities. In actively growing trees the number of leaves 
increases each year, and as all the foliage for a year is expanded 
during the early part of the growing season, additional vessels 
are needed at that time to supply the increased foliage. More- 
over, the vessels in the leaves formed during one season are 
directly connected with the wood produced that same season ; 
so, for this reason also, it is advantageous for the vessels to be 
formed as soon as possible, even when, as in trees past their 
prime, there is no increase in the number of leaves. In the case 
of coniferous trees similar conditions obtain, and the tracheids 
one season is sharply 
season’s growth. The — 
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