600 R. J. HARVEY-GIBSON AND MINNIE BRADLEY ON 



the stem. Dissection shows, however, that the number of bundles into which the 

 vascular system of the leaf resolves itself depends entirely on the size of the leaf, 

 that the forking of the central cord occurs very late, and that the interior strands 

 are by no meaus constant, many leaf-traces showing none at all. Fig. 32, a—c, shows 

 the most fully differentiated anterior strands that could be found, and it will be seen 

 that only those of the median bundle are developed. 



The course of the leaf-trace in P. somniferum follows no fixed rule, several 

 different methods having been noted. In no case was there any indication of the 

 formation of anterior bundles. Several variations were also met with in P. pilosum, 

 in which species also anterior bundles are absent.- The leaf-trace in P. orientale 

 consists of several strands which finally merge into three groups, the central consist- 

 ing of one bundle and the lateral of several. The anterior bundles are present, but 

 are only feebly developed ; these fuse into three arcs. These anterior bundles consist 

 only of phloem accompanied by laticiferous tubes. The leaf-trace enters by three 

 gaps in the axial system. Irregular fusion takes place between the axis bundles, 

 even the innermost series of strands being disturbed. On this point Leger's 

 account differs. 



The leaf-trace in P. Itevigatum consists generally of seven bundles, and anterior 

 bundles are occasionally found, and even then are feebly developed. Here also 

 variations in the mode of entry are frequent. 



In P. glaucium the leaf base largely ensheathes the stem (fig. 33, a-c), and the 

 leaf bundles finally unite into six, which give off small anterior strands. Further 

 fusions result in the formation of three main cords which pursue an independent 

 course down the stem. No anterior cords appear in young leaves. 



P. lateritium shows two types of leaf-trace entry with general features in common 

 but differing in detail. When a leaf has an axillary branch the entry is effected as 

 shown in fig. 34, a, b ; when there is no axillary branch the method is as shown in 

 fig. 35, a-c. In P. nudicaule the leaf-trace consists of one large median and two 

 large and several smaller lateral bundles. As the trace approaches the stem cylinder 

 the bundles approximate to each other. The median bundle enters first, followed by 

 the lateral bundles, and these gradually fuse in their passage down the stem. In the 

 peduncle, on the other hand, the leaf-trace bundles fuse into one cord before entry. 

 The same is true of the leaf-traces of P. alpinwm. 



The mode of entry of the leaf-trace in P. rupifragum is very similar to that met 

 with in P. Rhoeas, but as in that species variations occur even in the same plant. 

 When an axillary branch occurs the vascular tissue consists of a ring of isolated 

 bundles. These fuse, but leave a gap opposite the incoming leaf-trace, half the cords 

 of the axillary branch being placed on either side of the leaf-trace. In the leaf- 

 traces entering the stem near its base the anterior bundles are not so prominent nor 

 is the forking of the median bundle so well marked ; indeed, as a rule the two final 

 strands representing the leaf-trace rapidly unite into one broad bundle. Variations, 



