THE PHYSIOLOGY OF PLANTS. ^ 85 



on rapidly rotating wheels. If tlie rotation is vertical, the 

 centrifugal force is substituted for gravitation, and the roots 

 grow away from the centre of the wheel and the stems grow 

 toward it; if the rotation is horizontal, the centrifugal force 

 and gravitation act at right angles, and the roots will grow 

 in a line coinciding with a diagonal, or resultant of the two 

 forces, outward and downward, and the stems will grow up- 

 ward and inward. 



21. Certain well-known movements depend upon other ex- 

 ternal stimuli. Thus the leaves of many plants assume a posi- 

 tion at night (" sleep ") different from the ordinary or diurnal 

 position, in consequence of sensitiveness to light. For ex- 

 ample, the leaves of Clover, Vicia, Lathyrus, and Honey Lo- 

 cust fold upward at night ; those of the Locust and Oxalis 

 downwards. The petals of the Tulip, Oxalis, Portulaca, etc., 

 open and close alternately in the morning and evening, or 

 upon'a change of weather. The leaves of various species of 

 Oxalis, Mimosa, etc., are sensitive to contact and concussion. 

 The stamens of the Barberry, in contact with the corolla when 

 at rest, curve inward when lightly touched near the base, 

 bringing the anther in contact with the stigma. The stamens 

 of the Centaurea and other Composita; are sensitive to irrita- 

 tion. When at rest their free filaments, bearing united an- 

 thers, curve concavely outward ; on contact or concussion they 

 contract and straighten, lengthening again after some minutes, 

 and resuming their curved form. "This phenomenon occurs 

 only while the style is growing through the an4her-tube and 

 the pollen is being emptied into the tube. Th© motion of the 

 filaments effected by insects causes the antker^U^to be drawn 

 downward, and a portion of the pollerujl^ ^ escf^^ tlfcove 

 it, which is then carried away by insecre t^ Q^er fl^er^'and 

 capitula when the stigmas are alread^^o^d." (i^a(|Hs.) 



22. It often happens that growthj^il^esffllaog ^r^a^'dly 

 first on one side of an organ and^eiA)i<Cjh^o®br^d^ By 

 the alternating rate of elongatioift/ <^^ twQ^dsS, ^*move- 

 ment of nutation in one plane wil?[/ta^ pla^. ,3'^Mfy leaves 

 furnish a good example; in the Md^e g^t^^^Jwth is on 



o 



