MORPHOLOGY 



117 



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accordingly distinguished as one-strand or many-strand leaf-traces. 

 Sometimes a single vascular bundle becomes branched, and so appears 

 to be composed of more than one bundle. Eventually, however, each 

 bundle coalesces with another entering the stem from a lower leaf. 

 The arrangement of the bundles in a stem varies according to the 

 distance and direction traversed before the coalescence of the bundles 

 takes place. A relatively simple bundle arrangement may be seen 

 in the Eguisdaceae. In this family the leaves are arranged in alter- 

 nating whorls. From each leaf a one-strand leaf-trace enters the 

 stem ; at the next lower node each bundle bifurcates, and each half 

 coalesces with the bundles entering the stem from the leaves of that 

 node. This arrangement of the bundles may be shown diagrammatic- 

 ally, by representing the bundles as if on the surface of an unrolled 

 cylinder, so that they all appear in one plane. This is shown in 

 Fig. 131, and the connections of the bundles of the lateral branches 

 with the bundles of the parent stem are also 



shown (g). As the branches, in the case of 1 1 1 1 1 1 ' Jji. 



the Equisetaceae, alternate with the leaves, their V ^ /T 

 bundles on entering the stem are between 

 two leaf-traces of the same node, and at once 

 become fused with the leaf-trace which has 

 come from the leaf immediately above them in 

 the next higher node. The arrangement of the 

 bundles in the Yew (Tonus baccata), although 

 its leaf-traces have only one bundle, is much 

 more complicated (Fig. 132), for the bundles 

 maintain a distinct course throughout twelve 

 internodes before coalescing. Each bundle at 

 first descends in a straight direction through 

 four internodes ; it then curves to the side to 

 give place to a newly-entering leaf-trace, with 

 which it finally coalesces at the twelfth inter- 

 node. The position of a leaf necessarily deter- 

 mines the point of entrance of its leaf -trace into the stem, and 

 accordingly a diagram (Fig. 132) of the bundles of Taxus will exhibit 

 a divergence of the leaf-trace corresponding to the T \ divergence of 

 the leaves. The course taken by the leaf-traces in the stem, however, 

 is independent of the leaf position, and varies considerably in different 

 stems, although the divergence of their leaves may be the same. 



As a general rule, the leaf-trace bundles in Gymnosperms and 

 Dicotyledons arrange themselves in a circle in the stem. There 

 are, however, Dicotyledons in which the vascular bundles form two 

 (Phytolacca clioica, Piper) or more circles (Amarantus, Papaver, Thalictrum). 

 In such cases the inner circle is usually more or less irregular. 



In the stems of Monocotyledons (Fig. 124) the vascular bundles 

 are scattered, and without any apparent regular order. Their irregular 



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Fig. 131. —Diagram showing 

 the course of the vascular 

 bundles in the stem of 

 Equisetum arve'nsc. g, 

 Fusion of vascular bun- 

 dles of the lateral shoots 

 with those of the parent 

 shoot. 



