ADJUSTxMExXT TO GRAVITY, CONTACT, AND SHOCK 139 



proaches the base, the tip is carried beyond the vertical. Finally, 

 the tip swings back and is held in the normal position by the 

 fixing of the curvature in the base of the stem. In leaves the 

 region of curvature normally lies in the petiole, but if the leaf is 

 reversed and held firmly in some part of the blade, the free area 

 will attempt to twist into the normal position. In the case of 

 flowers, curvature normally takes place in the peduncle. 



i6o. Ecological significance of geotropism. Apart from the 

 fundamental action of gravity in maintaining the position and 

 form of plants, geotropism is of great value to plants in remedying 

 the effects of accidents due to wind, snow, animals, etc. In many 

 trees, especially conifers, geotropism brings about the replacement 

 of a broken or injured apex by one of the branches. Trees that 

 have been blown down sometimes regain an upright position, in 

 part at least, by means of geotropism, and they often convert the 

 branches of one side into upright stems. Herbs that have been 

 blown down by the wind or trampled by animals regularly regain 

 an upright position, at least in the new growth. Inflorescences, 

 flowers, and leaves that have been bent or caught in a mass of 

 leaves and branches, turn and twist to resume their normal i:)osition. 

 One not infrequently finds a stem hanging in such fashion that the 

 usual relations of the flowers are exactly reversed, and in conse- 

 quence the flowers, as well as the fruits, have curved about to take 

 their normal position. Finally, geotropism is a powerful factor 

 in the successful germination of seeds in nature by virtue of its 

 action in carrying the radicle into the soil. 



Experiment 44. Geotropism. Plant sunflower seeds in a pot, placing 

 some flat\vise, and others with either end downward. Cover the pot 

 with a wire netting to hold the soil in position, and invert it upon a 

 tripod, using a bell-glass to cover both. Explain the behavior of the 

 seedlings, and note the relation of the curvature to the original position 



of the seed. 



Transplant a sunflower seedling to each of six 2-inch pots. After 

 they are well estabhshed, place one in darkness in a horizontal position, 

 and one in the same position in light. For four successive mornings 

 put one of the remaining pots in a similar position. In the case of the 

 last two pots, restore the last plant to its normal relation after a half- 

 hour and note results. Remove the seedlings from the pot and sketch 

 them in order, showing the position of stem and root. Compare with 

 the plant grown in darkness Restore the last seedling to an upright 



