CHAPTER IV. MOVEMENTS OF PLANTS. 249 



a different position, in strong light, from that which they occupy 

 when the light is greatly diminished. These changes of position 

 are due to the movements of the protoplasm in which they are 

 imbedded. In strong light they ordinarily gather along the side 

 walls of the cell, or those which are perpendicular, or nearly so 

 to the surface of the organ, while in dim light or darkness they 

 congregate along the outer and inner walls. In Fig. 464, a is a 

 cell from the spongy parenchyma of the leaf of Oxalis acetocella 

 that has been exposed to weak or diffused day-light ; the corpus- 

 cles are nearly evenly distributed along the walls parallel to the 

 surface of the leaf, b shows a similar cell from a part of the leaf 

 which has been exposed for a short time to direct sunlight ; the 

 chlorophyll-corpuscles are here seen distributed along the walls 



a b . c 



Fig. 464. — Cells of the lowermost layer of spongy parenchyma from the leaf of Oxalis 

 acetocella, seen in a direction at right angles to the surface of the leaf a, plane position of 

 chlorophyll-corpuscles in diffused light : b, profile position after short exposure to the sun ; 

 c, position after longer exposure to the sun. After Sachs and Stahl. 



which lie perpendicular to the epidermis, c shows the position 

 of the corpuscles in a cell which has been exposed for a longer 

 time to direct sunlight. They are now massed together along 

 the walls perpendicular to the surface of the leaf. These proto- 

 plasmic movements are all closely related to the amoeboid 

 movements above described, and would doubtless result in similar 

 movements of locomotion were it not for the limitations imposed 

 by the cell-wall. 



(2) Geotropism. By this is meant those movements of 

 growth, the direction of which is determined by the stimulus 

 produced by gravity on the growing organs. It has been ascer- 

 tained by planting a germinating seed on the rim of a wheel 

 which was made to rotate in a vertical plane with a velocity 

 sufficient to produce a considerable centrifugal pull at the circum- 



