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), 
