GROWTH AND MOVEMENT 
479 
FIG. 699. Ordinary epidermis and "ocella 1 
(0) of leaf of Dioscorca. After HABERI.AMDT. 
in attaining these positions may involve curvature, lengthening, and 
twisting of the petiole and even of the blade itself. 
Perceptive region. Perception in most cases seems to occur in the 
blade, whence the excitation is propagated to the petiole, whose upper 
parts grow for the longest time, and even after elongation has ceased 
may be started into growth again by the light. In some cases, however, 
the petiole itself may be sensitive to light, and may either cooperate with 
the blade, or alone be responsible for both perception and curvature. 
The mechanism of perception has been sought in the epidermis of the blades. 
It has been found in some cases that the epidermal cells are domed and that they 
act as lenses (fig. 699), focusing the 
light upon the lower side of the cell, 
so that a spot in the center is much 
more brightly illuminated when the 
light strikes at right angles. The 
position of this area is shifted when 
the leaf blade is oblique to the rays. 
Correspondingly, it is assumed that 
the protoplast is excited when the 
bright spot rests on any but the central area. There is no doubt that the structures de- 
scribed concentrate the light, for that can be shown photographically ; but there are 
sensitive blades in which domed epidermal cells are wanting, and experiments do 
not yet unequivocally sustain the assumed distribution of irritability. The per- 
ceptive organs of leaves have not been located other than by this still doubtful 
hypothesis. 
(7) Other tropisms with radiant energy 
Electrotropism. Currents of electricity passing through the medium in which 
plants are growing, and presumably through the organs themselves, evoke various 
curvatures according to the density of the currents used. By nature roots lend them- 
selves especially well to experiment. Some of these responses, and possibly all of 
them, are due to one-sided injury of the roots. The effects appear to be due to elec- 
trolysis of the solutions used ; but whether by the direct action of the ions outside 
or by the withdrawal of ions from the protoplast is not certain. Electrotropism or 
galvanotropism may therefore be hardly more than a special form of chemotropism. 
It does not seem likely that such stimuli act to any important extent in nature. 
The more important effects of galvanic and static currents upon development have 
already been described (see p. 438). 
Thermotropism. Thermotropism is also of little importance. Both roots 
and stems of particular plants turn toward or away from a blackened plate radiating 
heat, according to the temperature. In a similar way roots growing in sawdust 
will grow toward or away from a source of conducted heat. Neither form of 
reaction can be of much importance in nature. 
The same may be said of reactions to radium and its salts, as well as those to 
X-rays. The injurious effects of these are more pronounced than the tropisms. 
