THE STEM AND THE LEAF 



53 



are rapidly dividing to form new ones, or, if not dividing, will 

 begin to do so whenever they are placed under favorable 

 conditions. Just back of the growing point appear little 

 protuberances (7) which are to develop into leaves. Further 

 along on the stem, each just forward of a rudimentary leaf, 

 are still more rudimentary branches (b v 5 2 , and so on). In 

 their youngest condition neither leaves nor branches con- 

 tain any fibrovascular bundles, but these soon appear, as 

 shown by the heavily shaded areas in the figure. Once 

 equipped with bundles for the transportation of water and 

 food materials, the growth of the young branch into a stem 

 like that from which it sprang, with bark, 

 wood, and pith of its own, is comparatively 

 rapid. Branches of trees, being structur- 

 ally of the same nature as the stem, form 

 " annual rings," just as the main trunk 

 does. The wood of the branch cuts across 

 the "annual rings" of the trunk and forms 

 a knot (Fig. 37). 1 



50. Internal structure of the monocoty- 

 ledonous stem. In the very young mono- 

 cotyledonous stem of seedlings the fibro- 

 vascular bundles are constructed like those 

 of dicotyledons, with the wood elements 

 on the one side and the cortical elements on 

 the other, as in Fig. 29. But in the full- 

 grown stems of most monocotyledons the 

 bundles have their vessels and other wood 

 elements arranged in a hollow cylinder 

 inclosing that part of the bundle which 

 corresponds to the portion shown outside 

 of the cambium ring in Fig. 29. In the 

 adult monocotyledonous stem (when it is solid) the bundles 

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



1 Knots may also be produced by injuries, but most of those found in 

 ordinary lumber were caused by branches. 



FIG. 37. Formation of 

 a knot in a tree trunk 



E, cut-off end of stick, 



showing annual rings ; 



K, knot, formed by 



growth of a branch 



