GROWTH AND MOVEMENT 423 



The growth of aerial stems is not hindered by the medium. When they 

 grow underground, the apex is protected by a cluster of overarching scales. 

 Growth of such stems is seldom rapid, but when it is, as in the extensive 

 running rootstocks of couch grass, the terminal bud is sharp-pointed and 

 smooth, so that .it offers the least resistance to being driven through the 

 soil; at the same time the firm scales protect the primary meristem 

 behind. In the root it is obviously advantageous to have the growth zone 

 restricted, and to have the zone of most rapid growth as near the apex 

 as possible; for, so much as any part behind it elongates, so far is the 

 tip actually driven through the soil. The sloughing and slimy surface 

 of the root cap lubricates the advancing apex, thus facilitating its pas- 

 sage. For good growth of roots (which makes for good growth above 

 also), it is desirable that the soil have an optimum content of water, 

 since it has been shown that its resistance to penetration is then at a 

 minimum. Drought, indeed, hinders root growth doubly; it not only 

 retards enlargement directly by lack of water, but also, by compacting 

 most soils, mechanically opposes the extension of the root system, and 

 so intensifies the difficulty of procuring the necessary water. 



Nutations. The rate of elongation is not only different in different 

 sections along the axis; it is also unequal in different segments around 

 the axis. This is especially marked in bilateral organs, such as leaves, 

 and varies from one face to another at different periods of development. 

 Thus, most leaves when young grow more rapidly on the back (later the 

 under surface), so that they are appressed to the stem; or they arch over 

 its apex when they outgrow it, as they commonly do, forming a " bud " 

 there. Later, growth becomes more rapid on the inner face (at matur- 

 ity the upper surface) and the bud opens. Local differences in rate lead 

 to the folding and rolling so characteristic, of young leaves in the bud. 

 In radially symmetrical organs, such as stems, inequality of growth 

 on different radii leads to bending, so that the tip is not erect but more 

 or less declined. As the most rapid growth shifts to different segments 

 around the axis, the tip nods successively to all points of the compass, 

 and so describes a very irregular ellipse or circle, or, considering also its 

 upward growth, a very irregular ascending spiral. Plotting successive 

 observations on a plane shows tracings like fig. 669. The nodding of 

 leaves or stems or roots on account of unequal growth is called nutation. 

 The inequalities in the rate of growth may be due to unknown causes, 

 assumed to be internal, when the corresponding nutation is called spon- 

 taneous or autonomic; or they may be due to external causes (stimuli), 



