GROWTH 
117 
tissue-tension. We are all familiar with this phenomenon 
(Figs. 74 and 75). When boys make whistles from young 
willow twigs in spring, a cylinder of bark is removed, 
and may be easily replaced; but if the cylinder of 
bark becomes split lengthwise, the edges cannot be 
made to come together around the wood without consider- 
able stretching. This illustrates transverse tissue-tension 
(Fig. 76). If the preceding statements in this chapter 
have been understood, the student should now be able 
to explain these phenomena without further assistance. 
FIG. 76. Portion of stem of a willow, illustrating transverse tissue- 
tension. By gentle tapping and twisting the cambium layer has been 
bruised, so that a small cylinder of the bark was easily twisted off. 
120. Elongation of Roots. In order to ascertain the 
manner of growth of roots, the root of a young seedling 
of lupine, or other plant, may be carefully marked with 
dots or lines of India ink, at intervals of i millimeter, 
beginning about i millimeter back from the root-tip, and 
extending for a distance of 15 to 20 millimeters. If the 
root is again observed, after having been left to grow for 
about 24 hours, it will be found that the first six or eight 
marks near the tip have spread apart, those from 3 to 7 
millimeters from the tip having separated more than those 
farther back Those marks most remote from the tip 
will be found to have separated very little if at all (Fig. 
77). By this simple experiment we learn that the 
