494 MODIFIED OIECUMNUTATION. Chap X. 



tation and heliotropism, we had the great advantage of 

 being able to lessen the light ; but with geotropism 

 analogous experiments were of course impossible. 

 We could, however, observe the movements of stema 

 placed at first only a little from the perpendicular, in 

 which case geotropism did not act with nearly so much 

 power, as when the stems were horizontal and at right 

 angles to the force. Plants, also, were selected which 

 were but feebly geotropic or apogeotropic, or had 

 become so from having grown rather old. Another 

 plan was to place the stems at first so that they pointed 

 30 or 40 degrees beneath the horizon, and then apo- 

 geotropism had a great amount of work to do before 

 the stem was rendered upright ; and in this case 

 ordinary circumnutation was often not wholly oblite- 

 rated. Another plan was to observe in the evening 

 plants which during the day had become greatly 

 curved heliotropically ; for their stems under the gra- 

 dually waning light very slowly became upright through 

 the action of apogeotropism ; and in this case modified 

 circumnutation was sometimes well displayed. 



Apoijeotropism. — Plants were selected for observation almost 

 by chance, excepting that they were taken from widely different 

 families. If the stem of a plant which is even moderately 

 sensitive to apogeotropism be placed horizontally, the upper 

 growing part bends quickly tip wards, so as to become perpen- 

 dicular; and the line traced by joining the dots successively 

 made on a glass-plate, is generally almost straight. For in- 

 stance, a young Cytimsfragrans, 12 inches in height, was placed 

 so that the stem projected 10° beneath the horizon, and its 

 course was traced during 72 h. At first it bent a very little 

 do\^^lwards (Fig. 182), owing no doubt to the weight of the 

 stem, as this occurred with most of the other plants observed 

 though, as they were of course circumnutating, the short down- 

 ward lines were often oblique. After three-quarters of an hour 

 the stem began to curve upwards, quickly during the first two 

 hours, but much more slowly during the afternoon and night, 



