380 



CONDUCTING SYSTEM 



f) 



a 



r\ 



\j 



3 



V 



over the entire cross-section of the stem ; in this way, differences in 

 the physiological activities of the several leaves are equalised. Nor is the 

 mode of origin of the vascular bundles which enter the axillary buds 

 altogether devoid of significance ; for it is evident that the buds do not 

 depend entirely for their nutrition upon their subtending leaves, since 



they receive plastic materials from members 

 of the next (higher) whorls as well. 



In order to illustrate the course of 

 the vascular bundles in the stem of a 

 Dicotyledonous plant, we may consider the 

 case of Stachys angustifolia described by 

 Nageli with the aid of the accompanying 

 diagram (Fig. 152). Here the leaves are 

 decussately arranged, or in other words, 

 placed in pairs with the median planes of 

 alternate pairs at right angles to one 

 another. The petiole of each leaf contains 

 a single bundle, which forks as soon as it 

 enters the stem ; the two shanks of the 

 leaf-trace diverge and, after passing down 

 through two internodes, unite at the second 

 lower node with the corresponding shanks 

 of the next leaf-trace below. A transverse 

 section across the stem, taken a short 

 distance below the apex, thus shows eight 

 bundles grouped in pairs in the corners. 

 If one of the larger bundles is traced 

 upwards, it will be found that the strands 

 that branch off from it pass out into leaves 

 belonging to two separate rows ; if, therefore, 

 we imagine the flow of material through 

 such a bundle to be interrupted or retarded 

 at any point, it is quite evident that the ill 

 effects of this disturbance will be evenly distributed among a number 

 of leaves belonging to two separate rows, and not concentrated upon 

 a single row, much less upon an individual leaf. 



In addition to such simple cases of bundle arrangement as the two 

 just described, a large number of much more complicated types have 

 been examined by Nageli and others. In spite of the straightforward 

 course of the individual bundles, the complications, in such cases, 

 may be very elaborate ; they depend for the most part upon the pre- 

 sence of multiple leaf-traces which become interwoven in diverse ways. 

 All the manifold varieties of bundle arrangement, however, clearly 



Fig. 152. 



Diagram showing the course of the 

 vascular bundles in the end of a shoot 

 of Stachys angustifolia [the cylindrical 

 surface being reduced to one plane). 

 ab, dc, fe, gh, ilc, traces of successive 

 pairs of leaves [the letters being placed 

 at the nodes]. Only one bundle of 

 each of the highest pair of leaf traces 

 is visible (i, k). After Nageli (from 

 De Bary, Comp, Anat.). 



