DICOTYLEDONOUS TYPE OF VASCULAR SYSTEM 379 



Juniperus nana. It should be noted at the start, that it is generally 

 considered preferable, in making a diagram of this sort, not to attempt 

 a perspective drawing, which is both troublesome to execute and 

 difficult to interpret, but rather to imagine the vascular cylinder split 

 open by a vertical cut and then laid out on a flat surface. 



The leaves of Juniperus nana are arranged in alternating whorls of 

 three ; the members of each whorl are not all inserted exactly at the 

 same level, but are slightly displaced spirally. A single vascular 

 bundle enters the stem from each leaf; this single leaf-trace descends 

 without dividing through an entire internode, but 

 towards the middle of the second internode forks 

 into two shanks, which insert themselves right and 

 left upon the adjoining leaf-traces. A transverse 

 section of a twig thus reveals either six or nine 

 vascular strands, according to the region of the 

 internode selected ; there will be six bundles 

 present, if the section is taken where the leaf- 

 traces are still undivided, whereas nine will be 

 met with further down, where the leaf- traces have 

 already forked (Fig. 151b). Every axillary bud 

 receives a pair of small bundles. 



If it be asked whether this very regular 

 reticulate arrangement of the leaf-traces serves 

 any useful purpose, the answer will most decidedly 

 be in the affirmative. It must be remembered 

 that each leaf-trace not only supplies the leaf to 

 which it belongs with water and mineral salts, but 

 also removes the surplus of plastic material manu- 

 factured in that leaf. If, now, one of the upper- 

 most bundles in the diagram be traced downwards, 

 it will readily be seen, that by the time it has 

 reached the third internode below the insertion 

 of the leaf to which it belongs, it has become 

 indirectly connected with every alternate member 

 of the entire circle of bundles, that is with 

 all. Precisely the same statement, of course, 

 other leaf-traces. As a result of the orderly arrangement of the 

 vascular strands, therefore, a proper supply of water and mineral salts 

 is secured to every leaf, while the effects of any inequality in the 

 development of the different bundles are practically neutralised through 

 being distributed over a considerable number of leaves. Similarly the 

 synthetic products of the leaves or at any rate that portion of them 

 which travels in the vascular tissues soon become uniformly distributed 



Fig. 151. 



A. Diagram showing the 

 course of the vascular bundles 

 in a twig of Juniperus nana 

 [the cylindrical surface being 

 reduced to one plane]. A-, the 

 strands supplying the axil- 

 lary buds. . B. T.S. through 

 a young shoot. 1, 2, 3, the 

 leaf -traces which pass out, in 

 the order of the numerals, 

 into the successive members 

 of a whorl. After Geyler 

 (from De Bary, Corny. Anat.). 



three leaf-traces in 

 applies to all the 



