124 



BOTANY 



the middle of tbe internode below, and the divisions diverge and 

 unite with the leaf-traces of the whorl below. The arrangement of 

 the bundles may be shown diagrammatically by representing the 

 bundles as if on the surface of an unrolled cylinder, so that they all 

 appear in one plane. This is done in Fig. 133, which also shows the 

 origin of the vascular bundles of the axillary shoots (k). 



The arrangement of the bundles iu the Yew (Taxus laccata), 

 although its leaf-traces have only one bundle, is much more compli- 

 cated (Fig. 134), for the bundles maintain a distinct course throughout 

 twelve internode 5 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 internode. The position 

 of a leaf necessarily determines the 

 point of entrance of its leaf-trace into 

 the stem, and accordingly a diagram 

 (Fig. 134) of the bundles of Taxus will 

 exhibit a divergence of the leaf-traces 

 corresponding to the yV divergence of 

 the leaves. The course taken by the 

 leaf -traces in the stem, however, is 

 independent of the leaf position, and 



FIG. 133. Diagram of the course of tin- . r . , , , . ,. 



vascular bundles in a young branch of varies considerably m different stems, 

 jnniperuf nnnn shown on the unrolled although the divergence of their leaves 



surface of the cylinder At I* the be the game The gtem Q f Ckmaiis 



vascular bundles passing to the axillary / 



shoots are seen. (After GEYI.KR.) vtiicella afford s an example of leaf traces 



consisting of three vascular bundles. 



The leaves are in whorls of two, the successive whorls alternating 

 with one another. The median strand of each leaf-trace (Fig. 135 

 ad, uk, nq, te) has a free course through one internode, and at the 

 node below divides into two arms which coalesce with the adjacent 

 lateral strands of the leaves inserted at this node. The two lateral 

 strands of each leaf-trace (Fig. 135 be, ef, hi, Im, op, rs) also are free 

 throughout the internode, but at the node below curve inwards and 

 become attached to the same lateral strands as the arms of the 

 median bundle of the trace. 



The traces of the axillary buds of most Gymnosperms and 

 Dicotyledons unite to form two strands which enter the mother shoot 

 and are inserted upon its leaf-traces (Fig. 133 k). 



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 

 (Cucurbita, Phytolacca, Piper) or more circles (Amarantus, Papaver, 

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

 irregular. 



