370 PRIMARY ARRANGEMENT OF TISSUES. " 



in gardens as Tradescantia albiflora, and will be described first in that example 

 (Figs. 119 and 120). 



From the sheath-like base of the alternate distichous leaves, which embraces the 

 stem, as a rule eight bundles curve into the node (i), and thence descend perpen- 

 dicularly to the next node (2). In the internode they are at about equal distances 

 laterally from one another, and at varying distances from the middle of the stem, but 

 at least ^ of the radius from it. Close above node (2) they curve towards one another 

 and the middle of the stem, and unite in the node itself in pairs to form four bundles. 

 These four bundles are stronger than the original eight : they are arranged crosswise 

 near to the middle of the stem, and run perpendicularly downwards to the next node 

 (3) where each inserts itself on the point of junction of two which pass out at node (2). 

 Each internode accordingly shows in transverse section (Fig. 1 20) in the first place 

 twelve bundles, four inner ones, arranged crosswise, and around them an irregular 

 circle of eight weaker ones [5). Besides these twelve bundles of the leaf-trace there 

 are also usually u-12 bundles (of subsequent development) which are arranged in a 

 circle outside the eight external bundles of the trace. This circle, together with the 

 rather small-celled parenchyma between the bundles, marks off the cortex from the 

 cylinder which contains the vascular bundles. Some of these bundles lie also further 

 inwards, between the eight outer bundles of the trace. These 1 1-12 bundles do not 

 pass out into the leaves, but run up into the youngest internode, passing almost per- 

 pendicularly through the internodes, curving slightly inwards at the nodes, and passing 

 near to the outgoing bundles of the trace. Irregular short transverse bundles connect 

 them in old nodes with one another, and with the bundles of the trace. 



Deviations from the above numbers are often found, e.g. in the lower internodes 

 of lateral shoots, where there are often in all only 18-19 bundles visible in the 

 transverse section — e.g. 3 + 6 bundles of the trace, and 10 cauline bundles. 



I found fundamentally the same arrangement both of the bundles of the trace 

 and of the cauline bundles in all the other Commelinaceae investigated : Commelina 

 agraria Kth., C. procurrens Schl., Tradescantia zebrina, virginiana, Spironema fragrans, 

 Dichorisandra thyrsiflora, D. oxypetala, Maravelia zeylanica. But in all these the 

 number of Ijundles of each category is higher than in Trad, albiflora, especially in 

 the last-named six species with a thick stem, and leaves with many bundles. The 

 arrangement of the separate and united bundles is accordingly more complicated and 

 requires further investigation. 



Potamogeion naians (Fig. 121) has alternating leaves in two rows: these are 

 often displaced from this arrangement (by torsion of the stem ?) : the leaf-trace 

 consists of three bundles, the width of the leaf-trace is about 180°. The three 

 bundles of the latter curve towards the middle of the stem, and pass separately down 

 one internode, the stronger median bundle being nearer the middle line than the two 

 lateral ones. In the next node they all three coalesce to a single bundle, which then 

 passes down to the second node, and here inserts itself at the point of coalescence of 

 the next lower trace (rafely one of the lateral bundles continues a separate course up 

 to this pbint of insertion, Fig. 121, x). In the internode there appears accordingly 

 in the bluntly rectangular transverse section (Fig. 122) of the 'cylinder' which con- 

 tains the bundles, one large bundle at each end of the smaller diameter ; these are 

 opposite one another: one of these (i) is the united trace of the seQond higher leaf, 



