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 

 lie. taken into consideration, namely, that light retards growth. 

 A further instance is that of plants 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 (Passifiora 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. 



