370 BowMAN: MECHANICAL TISSUE IN CERTAIN VINES 
However, as the secondary thickening is not considered in this 
study, its details will not be mentioned. A peculiar though common 
phenomenon is seen in the Tecoma in that there is a great increase 
in the parenchymatous tissues on the side from which adventitious 
roots or holdfasts are put out, so that there is a great differential 
growth seen in transverse section. 
In the Rosa, which is a ‘‘leaner’’ or ‘‘weaver”’ according to 
Kerner von Marilaun’s terminology, the vine habit is almost lost. 
This is like the Hedera inasmuch as it has so much plasticity that 
by proper culture it can be made to assume a tree-like or shrub-like 
orm. The mechanical tissues are well represented, the large, 
thick-walled fibers form an interrupted sheath in the transverse 
section and there are also many large crystals of calcium oxalate 
in the peripheral region; the collenchyma is very thick-walled and 
the xylem is arranged in numerous narrow bundles separated by 
frequent, though attenuated medullary rays. The Rubus shows 
very similar characters. This genus is also included in Kerner von 
Marilaun’s weaving category, but it has various species in southern 
Asia and Australia which are typical lianas. In both Rosa and 
Rubus, of course, the pith area is very large in cross section. In 
the Rubus the bast fibers form a heavy belt and the collenchyma 
too is developed to six layers of cells. The last species studied, 
Kerria japonica, is a weak shrub and never develops any vine habit. 
In this plant, however, the mechanical support does not seem to 
come so much from the bast fibers, etc.,as from the principle of 
construction, i. e., a very firm outer cylinder filled with pith. The 
bast fibers are large but few in number and scattered in small 
groups, the perennial epidermis doubtless also contributing to the 
stability of the stem as also the lignified tissues of the a broad, 
flattish vascular bundles. 
Now in looking over the anatomy of this group of plants it 
would seem that the variation in stereome elements in stems would 
naturally be a factor of great importance in the evolution of the 
vine habit. Of course the first and most important agent is light, 
since light by its action on the chloroplast tends to change the 
shape of the cell. Diffuse light is known to cause an elongation of 
parenchymatous cells and this of course produces elongation in an 
organ. This then partially accounts for the elongated habit of 
