THE STEM. 45 
with the soil. The heliotropism of the stem is, again, opposed 
to that of the root, for similar reasons. It is, except in special 
cases, positive. Light is essential to leaf functions; and many 
plants which in open localities have comparatively short stems 
become extremely elongated when surrounded by a dense under- 
growth. The excessive development in length of shoots grown 
in a dark or badly-lighted place may also be interpreted as an 
effort to reach the light. The sprouting of potatoes in damp 
cellars is a familiar example. There is, however, another fact to 
be taken into consideration, namely, that light retards growth 
in length, as seen where plants are grown in a window. ‘These 
curve over towards the light to such an extent as in many cases 
“‘to break their backs.”’ The side turned towards the room here 
grows more rapidly than the other side in its attempt to reach 
the light, with the result mentioned. Climbing stems do not 
exhibit strong positive heliotropism, as this would often take 
them away from their supports. They may even be negatively 
‘heliotropic. This is seen in the tendrils of the Virginia creeper, 
which turn away from the light and attach themselves to the 
wall up which the plant climbs. Again, stems are often very 
sensitive to mechanical contact, and this is particularly the case 
with stem (and other) tendrils. The best example is a kind of 
passion-flower (Passijflora gracilis), where, according to Darwin, 
a perceptible curving takes place half a minute after the tip 
is lightly touched. When, therefore, such a tendril comes into 
contact with a support, it is enabled to attach itself very quickly. 
A very interesting case is that of the white bryony (Bryonia dioica). 
The long tendrils of this plant coil their ends round supports, and 
this affects the unattached parts of them near the stem, causing 
these to coil up like corkscrews. Two ends are served. The 
plant is pulled upwards and firmly stretched, and the coiled parts 
in question act like so many springs, which yield in a strong 
wind, and prevent it from tearing the stems from their support. 
Fia. 8*.—Diagrams of Sieve Tube. A,. longitudinal section, cell-walls unshaded, 
lining protoplasm black. B, cross-section on a b, showing perforated sieve-plate. 
‘C, section on ¢ d, showing cell-wall, protoplasm, and central vacuole. 
