68 INTRODUCTION TO BOTANY 
As the branch lengthens, its wood cuts across the annual 
layers of the stem from which it grows, and the branch forms 
its own annual rings. Knots are not all due to the growth 
of branches, but most of them are, as may be seen from 
figure 46. If a knot-forming branch dies early, new wood 
forms over it and covers it up, as the figure shows; but if it 
continues to live as long as the main stem does, it gives rise 
to a knot that reaches to the outer- 
most layer of wood in the stem. 
In figure 46 which knot, a or 6, 
would be the more likely to in- 
jure the timber ? 
66. Internal structure of the mon- 
ocotyledonous stem. In the very 
young monocotyledonous stems of 
seedlings the fibrovascular bundles 
are constructed like those of di- 
cotyledons. The wood elements 
Fic. 47. One quarter of a cross 
section of a corn stem, showing : : 
the hard cortex and within it ate on the one side and the corti- 
the soft pith, throughout which cal elements on the other. In the 
many fibrovascular bundles are 
‘ full-grown stems of most mono- 
irregularly scattered 
cotyledons the bundles have their 
vessels and other wood elements arranged in a hollow cylinder 
inclosing that part of the bundle which corresponds to the por- 
tion shown. outside of the cambium ring in figure 42, 4. In 
the adult monocotyledonous stem (when it is solid) the bun- 
des occur scattered all through the pith, as shown in a section 
of asparagus or corn stem (fig. +7). No such complicated bark 
as that of woody: dicotyledons is found in monocotyledons. 
67. Growth in thickness of the monocotyledonous stem. ‘The 
very young stems of monocotyledons may for a time increase 
considerably in diameter by the formation of new bundles 
within them. But in monocotyledons all the cambium becomes 
changed into other tissues, so that none is left, as it is in 
dicotyledons, to develop new tissue. In monocotyledons the 
bundles are said to be closed, while those of dicotyledons, in 
