520 Shove.— On the Structure of the 
of interest may be mentioned with reference to the structure 
of the vascular tissue in the leaf-bases. 
The method of division of the initial leaf-strands into a net¬ 
work has been already described, and the superficial part of 
the network is shown in Figs. 32 and 33. Fig. 33 represents 
the superficial tissue from the inner flat side of the leaf-base; 
the strands enclosed within the space a , b, c, d , are those 
which occupy the connate part of the stipules, and those at 
/, q> r i s , are branches making their way inwards to the petiole 
proper ; the other petiolar bundles arise from the inner surface 
of the superficial vascular tissue and are not therefore repre¬ 
sented in the diagram. 
Fig. 32 shows the superficial vascular strands from the 
outer convex surface of the leaf-base ; these are not repre¬ 
sented in perspective, but as spread out over a plane surface. 
The cut ends of the bundles represented as lying along the 
arc ab are those of branches destined for the petiole ; it is 
obvious that the petiole is thus continuous with the convex side 
of the leaf-base. At S are shown the fine strands belonging 
to the stipules ; these form a network with long drawn-out 
meshes, and are accompanied by numerous secretory passages. 
The secretion, probably tannin, is contained in sacs or cells 
arranged in longitudinal rows; these are seen particularly 
well in a median section of the root-tip. 
In the leaf the structure of the stele is that of the 
ordinary stem type, with the exception of the absence of 
the arc of protophloem. As in the stem, the vascular elements' 
first differentiated form an arc of sieve-tubes on the side of 
the stele furthest from the centre of the organ, but in the 
case of the leaf these are of considerable size, and later form 
part of a continuous ring. 
Soon after the formation of the protophloem, the protoxylem 
appears along the opposite side of the stele, one group only being 
formed in the case of the smaller, semi-lunar-shaped steles. 
The ultimate bundles ia the stipules show no recognizable 
phloem, but consist of a few tracheides, surrounded by small- 
celled parenchyma; frequently one tracheid only is present 
in the stipular steles. 
