UNEQUAL GROWTH. 
857 
These movements of nutation of bilateral appendicular organs take place mostly 
in one plane which coincides with the median plane of the organ. As long as the 
organ grows most rapidly on the dorsal side, it may be termed, after de Vries, 
hyponastic ; afterwards, when it grows most rapidly on the inner or upper side, 
epinastic. Since in the later stages of development of an organ growth ceases at 
certain places — while at different distances from these places it presents different 
stages of growth, until it finally ceases everywhere — it is clear that in the same organ, 
together with areas where growth is completed and nutation no longer takes place, 
others occur with hyponastic and others again with epinastic growth, until at length 
nutation and growth alike cease altogether, as in Fern-leaves. 
Seedlings of Dicotyledons afford a remarkable illustration of bilateral structures 
which nutate in one plane ; although their stem and primary root become afterwards 
multilateral and grow vertically upwards and downwards. The stem terminates in 
a pendent or nodding bud ; and the curvature, which is generally very great, exhibits 
itself also in germination when it takes place out of the ground in a vessel that 
rotates slowly round a horizontal axis ; it is a true curvature of nutation inde- 
pendent of light and gravitation. But the older portions of the stem become straight 
as they develope from the curved portion ; and in proportion as the stem increases 
in length, the straight part which bears the nodding bud also lengthens. When 
germination takes place in a feeble light, or better in a slowly rotating vessel, a 
more rapid growth occurs of the side of the older portion of the stem which was 
at first concave, causing it to become convex; and hence the older and younger 
parts of the stem form together a letter S, as in Phaseolus, Vicia Faha, Polygonum 
Fagopyrum, Cruciferse, &c. But the primary roots of dicotyledonous seedlings also 
manifest a tendency to a bilateral organisation ; since, when they develope under slow 
rotation round a horizontal axis, they seldom continue to grow straight, but curve 
concavely either in front or behind, sometimes even becoming rolled up. These and 
other instances of nutation are not clearly seen when the development takes place 
under normal conditions, because the growth of the stem of the seedling is retarded 
by light, and the curvature both of stem and root prevented by geotropism. 
A knowledge of the different capacity for growth possessed by the anterior and 
posterior sides of bilateral organs lies at the root of an understanding of the fact that 
leaves, lateral shoots, and many secondary roots, although they are heliotropic and 
geotropic, yet assume definite positions with respect to the horizon, but without 
growing vertically upwards or downwards. When multilateral (orthotropic) primary 
stems and roots grow vertically, the essential cause is their growth being uniform on 
all sides of the axis of growth ; the different sides of the organ are in equilibrium 
with one another. Every deviation from the vertical position, to the right, left, front, 
or back, is counterbalanced by geotropism ; the growing part curves until the free 
apex stands erect, in which position the action of gravitation is again equal on all 
sides. In the same manner light acts equally strongly on all sides of such organs. 
If therefore one side is exposed to stronger light, a heliotropic curvature takes place 
which finally brings the free part into a position in which all sides receive equally 
strong light on all sides, and therefore grow uniformly without any further curvature. 
The case is different with bilateral organs the anterior and posterior sides of which 
possess independently different capacities for growth (plagiotropic), and which there- 
