3 6 
GROWTH 
complete the organ assumes the straight line. Nutation 
is most marked in climbing plants (Chap. III.). 
Irritability ; Movements ; Orientation. Living 
protoplasm is sensitive or irritable to many stimuli. In 
higher plants, this sensitiveness is chiefly shown in movernents 
of growing organs which tend to grow in such a way as 
to take up definite positions (or orientate themselves) with 
regard to the incidence of the stimuli acting upon them. The 
orientation generally depends on the symmetry; a radial 
organ usually places itself in line with (or is orthotropic to) 
a stimulus, a dorsiventral organ at right angles ( plagiotropic ) 
to it. If the direction of the stimulus change, the growing 
point curves to reassume the proper orientation, as is seen 
in plants curving towards light in a window. Two cases of 
orthotropism must be distinguished, positive (growth towards) 
and negative (growth away from the stimulus). The different 
irritabilities have special names, viz. heliotropism (light), 
geo tr op ism (gravity), hydrotropism (moisture), chemotropis7n 
or chemotaxis (chemical stimulus), rheotropism (water cur- 
rent), &c. Sensitiveness to contact stimulus is found in 
some cases, but almost solely in climbing plants, and there 
more especially in tendrils (see Chap. III.). 
An organ is described as helioiropic or positively heliotropic 
if it grow directly towards light, negatively or apo-heliotropic 
if directly away from it, plagio - or dia-heliotropic if at 
right angles to it ; similar expressions are used for the other 
irritabilities. 
Movement of mature organs is found chiefly in leaves 
(see below), and in some stamens, flowers, &c. (see Flower- 
mechanisms, below). 
The Root. The ‘true 7 root of a higher plant is the 
organ descended from the original absorptive organ first 
differentiated from the shoot. In the vast majority of these 
plants it is present in the embryo, starting at one pole while 
the shoot starts at the other. Its structure may be studied 
in seedlings of beans, gourds (fig. 1 ), mustard, &c. The 
embryo plant in the seed has a short shoot-axis or hypocotyl , 
at the upper end of which are two seed-leaves or cotyledons 
and a small stem-bud or plumule , while at the lower end 
there is a short radicle , which grows out to form the primary 
root. In systematic descriptions the term radicle generally 
