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 5, 

 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 

 are on the one side and the corti- 

 cal elements on the other. In the 

 full-grown stems of most mono- 

 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, A. In 

 the adult monocotyledonous stem (when it is solid) the bun- 

 dles occur scattered all through the pith, as shown in a section 

 of asparagus or corn stem (fig. 47). 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 dosed, while those of dicotyledons, in 



FIG. 47. One quarter of a cross 

 section of a corn stem, showing 

 the hard cortex and within it 

 the soft pith, throughout which 

 many fibrovascular bundles are 

 irregularly scattered 



