Elementary botany . night positions which theleaves assume, there is a pair of small lateral leaflets to each leaf which con-stantly execute a jerky motion, and swing around in a circle like the secondhand of a watch. 4oo RELATION TO ENVIRONMENT. ulus were entirely removed. As we know also contact or pres-sure stimulus, or jarring produces the same result in sensitiveplants like mimosa, some species of rubus, etc. In many plantsthere is no well-developed pulvinus, and yet the leaves showsimilar movements in assuming the day and night are seen in the sunflower, and in the cot
Elementary botany . night positions which theleaves assume, there is a pair of small lateral leaflets to each leaf which con-stantly execute a jerky motion, and swing around in a circle like the secondhand of a watch. 4oo RELATION TO ENVIRONMENT. ulus were entirely removed. As we know also contact or pres-sure stimulus, or jarring produces the same result in sensitiveplants like mimosa, some species of rubus, etc. In many plantsthere is no well-developed pulvinus, and yet the leaves showsimilar movements in assuming the day and night are seen in the sunflower, and in the cotyledons ofmany plants. A little observation will enable any one interestedto discover some of these plants.* In these cases the nightposition is due to epinastic growth, and while this influence isnot removed during the day the light stimulus overcomes itand the leaf is raised to the day position. 770. Leaves which rotate with the sun.—During the growthperiod the leaves of the sunflower as well as the growing end. Fig. 439-Same sunflower plant photographed just at sundown. of the stem respond readily to the direct sunlight. The re-sponse is so complete that during sunny days the leaves towardthe growing end of the stem are drawn close together in theform of a rosette and the entire rosette as well as the end of the * Seedlings are usually very sensitive to light and are good objects tostudy. FOLIAGE LEAVES. 401 stem are turned so that they face the sun directly. In the morn-ing under the stimulus of the rising sun the rosette is formedand faces the east. All through the day, if the sun continues toshine, the leaves follow it, and at sundown the rosette facessquarely the western horizon. For a week or more the youngsunflower head will also face the sun directly and follow it allday as surely as the rosette of leaves. At length, a little whilebefore the flowers in the head blossom, the head ceases to turn,
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