CHAPTER VII. — SECONDARY GROWTH IN THICKNESS OF 
STEMS AND Roogs. 
THE process known as secondary growth in thickness, is 
common to the stems and roots of most dicotyledons and gymno- 
sperms. It occurs also in some of the larger stems of the mono- 
cotyledons, but their normal stem is not planned to allow for 
yearly increase in radial diameter, and the stems which do admit 
of such increase may be considered as having changed from the 
mono- to the dicotyledonous type. ‘The origin of the bundles 
in the dicotyledonous stem and their manner of distribution 
have already been given while describing the elements of the 
vascular system. We may now return to the same figure used 
there for illustration, namely, a cross-section of an ordinary . 
dicotyledonous stem. ‘The bundles as a rule are collateral, open, 
and all leaf-traces ; their course in the stem 1s such that if the 
entire section containing them, were taken out from the stem, 
or separated from other parts of it, this section would be a 
hollow.cylinder. There would remain two other parts, a central 
solid cylinder or pith, and an ‘outer hollow cylinder or rind. 
Growth in thickness of such a stem can be accomplished through 
the activity of the cambium cells lying in the cylinder contain- 
ing the bundles, but only on one condition, that is, that the 
ground cells lying between the bundles also increase in radial 
diameter either by the growth of the cells already present, or 
by some of them becoming meristematic and so forming new 
celis. In stems designed to live for some time, the latter method 
is followed by the formation of the so-called cambium ring.} 
1 As there is so little uniformity among authors in the use of anatomical 
terms, for convenience, we have used the word cambium to denote only the 
prosenchymatic meristem, in order to distinguish it from the parenchymatic. 
