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- 4 
BOWMAN: MECHANICAL TISSUE IN CERTAIN VINES 369 
The pith and the parenchyma of the cortex also occupy relatively 
large spaces and contribute nothing to support. In this plant the 
characteristics of the ivy are again reviewed. If the Lycium is 
grown in open situations, such as fields, the plants can make a 
growth of several meters before becoming entirely prostrate, and 
if pruned will form a dense, shiny shrub. If it grows in diffuse 
light near a support, however, the characteristic “leaner” habit 
or, as Kerner von Marilaun calls it, the ‘‘weaving’’ habit, is 
developed. 
The Psedera hasa very definite arrangement of the mechanical 
tissues. The xylem occupies almost a continuous cylinder outside 
the pith, the rays separating the bundles being very narrow. In 
the wood cambium the bast fibers conform to the type called by de 
Bary branched sclerenchymatous fibers, forming a continuous 
cylinder with a thickness of six cells. This makes the stems very 
tough and by experiment in the laboratory it was found that a 
stem of one season’s growth, 70 cm. long and 0.3 cm. in diameter, 
bore a strain of 8845.2 grams before breaking but could not 
support an erect position in a stem of the same dimensions more 
than 70 cm. in length. These sclerenchymatous fibers in trans- 
verse action measured 0.06-0.08 mm. in their greater dimension and 
0.0I—0.02 mm. in the shorter, the sections being irregularly oblong; 
the lumina were about 7 u wide, thus showing the walls to be com- 
paratively heavy. According to the above experiment, however, 
the fibers do not suffice to maintain the stem in an upright position. 
The arrangement in the Vitis is similar to the preceding in a 
great many respects. The xylem is here developed to a greater 
extent and this gives a considerable support. The medullary 
rays are clearly defined and also the cambium ring. Inthe Tecoma 
there is very good reason for the toughness of the stem. In 
transverse section, by measurement it was seen that in a belt of the 
cortical parenchyma 0.9 mm. wide there were thickly scattered 
groups of short sclerenchymatous fibers and also a heavy rein- 
forcement in the external layer of collenchyma. The stem, too, 
occupies a large area. In fact, in the Tecoma the vascular elements 
occupy 3314 per cent of the stem and the secondary thickening is, 
of course, very interesting. Haberlandt (4, p. 629) remarks that 
Sanio first observed that the wood and bark formation took place 
in the reverse order from most plants. 
