100 ELEMENTS OF PLANT ANATOMY. 
the stem increases in diameter, the one-third leaf position 
changes to three-eighths, and the net-formed tube begins at the 
insertion of the last leaf of the one-third arrangement. 
This may be illustrated by a diagrammatic sketch. In 
Fig. 43 let the shaded bands represent the vascular tissue, and 
the white or unshaded portions the meshes; the points of 
insertion of the leaves will then be at the places indicated by 
a, 6, c, ete. The leaf whose 
insertion occurs at a has a 
single bundle bending out 
at this point, which is formed 
by the union of two others 
originating at the points 6 
and ce, or the points where 
the bundles of the leaves 4 
and e¢ bend outward into 
them, their position in re- 
ference to point a being 
always that of the two next 
older leaves on either side. ~ 
The repetition of this forms 
a net with rhomboid meshes. Each leaf receives also four to 
six bundles from along the sides of the network. In large 
and well developed plants, each leaf receives therefore as many 
as seven bundles, one at the lower angle of the mesh and six 
above, three on each side, as indicated at y. In the next year 
the stem becomes much thicker, the leaf position going over 
to five-thirteenths, where it usually remains. 
The bundle system in the foregoing example is considered 
as made up of leaf-traces, and this is also true of that of many 
other ferns having this cylindrical network. In many kinds of 
fern-stems, however, it is impossible to ascertain the exact 
relation between leat and stem bundles; while in others it is 
decided that the network, otherwise similar to that described, 
Fia. 48. 
Explained in text. 
