OHBMISTRT AND PHTSIOS 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 a long 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. 



(S) Germinate and study in lilce manner the seeds of cabbage, rad- 

 ish, Indian corn. 



(e) Fix a slender filament of glass to the rapidly growing end of a 

 shoot of fuchsia, geranium, or verbena (using a drop of tliick shellac- 

 glue), and observe tlie 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. Or a 

 microscope may be fixed in such a position that the tip of the pointer 

 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 approximately 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. 



(/) Observe the sleeping state of wood-sorrel (Oxalis), clover, and 



