CHEMISTRY AND PHYSICS OF PLANTS. 95 
is called) and heliotropism have then this in common, that 
both have as their basis that continual. movement of the 
plant which appears to be the constant accompaniment of 
life; in the one case this movement receives special direc- 
tion and impulse from the light, while in the other the im- 
pulse is given by the force of gravitation. 
197. We may now also connect the movements due to 
ordinary mechanical stimuli with the foregoing. In the 
well-known sensitive-plant a slight touch or jar is sufficient 
to cause the leaves to close with considerable rapidity. 
This was for along time referred to an obscure irritability, 
which was regarded as something peculiar to a few plants. 
If, however, we bear in mind that motion appears to be the 
normal state of growing parts, or parts whose tissues re- 
main thin-walled, we see that this “irritability” is not a 
peculiarity at all, but only an intensification of that which 
is possessed by plants in general. 
Practical Studies.—(a) Soak a few beans in water, and when the 
little roots begin to protrude pin the beans carefully to a weighted 
cork under a bell jar, and observe the movements of the radicles. 
(0) Germinate and study in like manner the seeds of cabbage, rad- 
ish, Indian corn. 
(c) Fix a slender filament of glass to the rapidly growing end of a 
shoot of fuchsia, geranium, or verbena (using a drop of thick shellac- 
glue), and observe the circumnutation. If a plate of glass be laid 
horizontally just above the tip of the glass pointer, the movements of 
the latter may be readily recorded by lines or dots on the glass, Ora 
microscope may be fixed in such a position that the tip of the poiuter 
is in focus, when the movement will be made visible to the eye. 
(d) Fix a glass pointer to the tip of a leaf of a suitable plant (as a 
fuchsia, geranium, primrose, etc., grown in-a pot), and record the 
nutations on a glass plate fixed vertically or horizontally in such a 
way as to be approximatcly at right angles to the pointer. 
(€) Germinate seeds of cabbage, radish, parsley, or tomato, and note 
carefully the position of the cotyledons during the day and night. 
(f) Observe the sleeping state of wood-sorrel (Oxalis), clover, and 
