368 BOWMAN: MECHANICAL TISSUE IN CERTAIN VINES 
ical tissues are represented only by the xylem and the small 
amount of bast next to the outer cortex parenchyma. A peculiar 
feature of this latter is that under certain conditions it is greatly 
increased and bush forms of the Hedera can be noted in many 
gardens. The occurrence of this form as a variety is frequently 
noted in botanical literature (see de Vries, 8, vol. 1, p. 44). 
This would go to show that in the ivy, if it grows as a bush, i. e., 
if it is well fertilized and pruned, the bast elements will develop 
to such a degree that it can maintain an erect position. The 
shock of pruning seems to cause this undue development of bast. 
This plasticity of the stereome development offers ground for the 
theory that the liana habit arose from erect plants. 
In the Rhus there are almost no medullary rays to be clearly 
distinguished, the primary xylem is well developed and in the 
phloem the resin ducts are seen, as mentioned by de Bary (1, P- 
452). The mechanical tissues are poorly developed in primary 
growth except the collenchyma, of which there are approximately 
three layers of cells. In the secondary growth the bast fibers are 
parallel in development with the secondary wood. In the Rhus 
the estimate is 39.51 per cent for the vascular elements. 
The Wisteria shows the greatest development of sclerenchy- 
matous fibers with very heavy walls, so that they appear almost 
solid. In the wood cambium and the cortical parenchyma there 
are a great many calcium oxalate crystals and in the latter region 
are also many irregularly shaped stone cells. These give the plant 
considerable strength, but there are not enough of the long fibers 
to produce a solid cylinder. A young plant with this amount of 
mechanical tissue can keep an erect position until it is a half meter 
high, when the weight of the increasing growth becomes too great 
for these tissues and it assumes a recumbent position. This was 
observed in young plants grown under various conditions and in 
different situations. The leaning position is assumed much earlier 
in diffuse light. 
There are peculiar conditions in the mechanical development of 
the Lycium. This plant is half shrub and half vine. Its manner 
of growth is most interestingly discussed by Kerner von Marilaun 
(6, vol. 1, p.672). The xylem occupies a very large space and the 
bast is developed so as to fill 18 per cent of the transverse area. 
