STEMS AND BRANCHES AND THEIR USES . 187 



ent conditions, one may be short and stocky, and the other 

 tall and slender. The stems of some plants remain short for 

 a time, because their internodes do not grow in length to any 

 extent ; then comes a period when the new internodes grow 

 rapidly, forming a long, slender part of the stem. An ex- 

 ample of this kind of growth is seen in the radish, whose 

 foliage leaves are borne close together on the short lower part 

 of the stem ; the long upper part, that grows much later, 

 bears the flowers. There are some plants whose nodes grow 

 in length instead of the internodes ; in the arbor vitas (white 

 cedar) the growth of the nodes stretches the bases of the 

 leaves out along the stem ; since the internodes remain short, 

 the leaves are close to one another. 



207. Nutation. In a stem whose internodes continue 

 to lengthen for some time, the rate of growth is not the same 

 on all sides. At any particular time, growth is most rapid 

 on one side of the stem and so the stem bends toward the 

 opposite side. At a later time the region of most rapid 

 growth is on a different side, and the stem is bent in another 

 direction. The point of most rapid growth moves, as it were, 

 more or less regularly about the stem. As a result, the 

 direction in which the stem is bent is continually changing, 

 and the tip of the stem swings around. Since the stem is 

 steadily growing longer and pushing its tip forward as well 

 as swinging it about, the movement of the tip is on the whole 

 a spiral one. This movement of the stem tip is called nuta- 

 tion. Usually each internode, as it becomes older and ceases 

 to grow, straightens so that the stem is not permanently 

 twisted. Nutation is especially marked in climbing plants 

 with twining stems, such as the bean and the morning glory. 

 It is partly because of its nutation l that a bean stem winds 

 about any slender object with which it comes in contact. 

 At first the coils of the stem are loose ; but as each internode 



1 The stimulus of gravity also plays a part in bringing about the winding of a 

 bean or other twining stem. 



