110 



CIRCUMNUTATION. 



painted gold-beater's skin the stems bent to 

 the same degree as those which were not cov- 

 ered. Shielding the tips of older seedlings with 

 the same opaque coverings did not prevent the 

 plants from bending toward the light. The 

 remarkable power of transmitting an influence 

 to the adjoining parts resides, therefore, in the 

 tips of very young sprouts as well as in the 

 extremity of the radicle. It was discovered, by 

 amputating the tips of cotyledons, that the ef- 

 fect of apogeotropism is not derived from them, 

 as geotropism is from the tip of the radicle. 



The stems of a number of developed plants 

 of widely different families were found by Dar- 

 win to circurnnutate slightly while growing. 

 The ellipses described were usually narrow, 

 though not so narrow as those of stolons or 

 leaves, and are irregular in their forms and in 

 their periods. A stem will sometimes describe 

 one ellipse on one day and two the next. Some 

 plants make several irregular revolutions with- 

 in a few hours. The axes of the different 

 ellipses described point in all directions; the 

 stem, after bending as far as it goes in one di- 

 rection, in its returning path bends toward a 

 point nearly but not exactly opposite. 



The circumnutation of stolons or runners is 

 of much greater amplitude than the regular 

 circumnutation of stems, and is suspected by 

 Darwin to be a special adaptation. They cir- 

 cumnutate in a very complex manner. The 

 movement is usually in a vertical plane, owing 

 to the weight of the runner ; but there is al- 

 ways more or less lateral motion. A straw- 

 berry-runner moves up and down many times 

 in a day, besides some lateral movement. The 

 distance traveled by the end of a stolon during 

 fourteen hours was found by measurement to 

 be 2*67 inches. The movements of stolons are 

 in a great measure opposed to the force ot 

 gravity. Such remarkable amplitude of mo- 

 tion in the circumnutation of running plants, 

 which ran only be compared to that of climbers, 

 must be exceedingly beneficial to them, en- 

 abling the running branches to surmount ob- 

 stacles which would otherwise contort and 

 cripple them. The remarkable amplification 

 of the circumnutating movement in the stems 

 and in the tendrils of climbing plants can be 

 more clearly ascribed to adaptation. Their 

 steins while young circumnutate in the ordinary 

 manner. After a while they begin to elongate 

 rapidly, and their power of movement becomes 

 enormously increased. The circumnutation is 

 more even and regular than in the case of 

 other plants. The tendrils also, whether con- 

 sisting of modified leaves or of modified flower- 

 peduncles, sweep wide circles in their circum- 

 nutation ; whereas in ordinary leaves the move- 

 ment is generally in a vertical plane. 



Flower-peduncles while growing manifest 

 the property of circumnutation. The leaves of 

 plants belonging to both the dicotyledonous 

 class, gymnosperms as well as angiosperms, and 

 to the dicotyledonous class of phenogamous 

 plants, and also those of the cryptogamic sub- 



kingdom, have been proved to circumnutate. 

 The seat of the movement is commonly in the 

 petiole ; but sometimes it lies in both petiole 

 and blade, and sometimes in the blade alone. 

 The movement is always a very slight one. 

 The angle passed through varies from 2 to 10 

 in the generality of plants, but in the bean it 

 amounts to 23. The main movement is in 

 a vertical plane, but with slight lateral devia- 

 tions, producing ellipses of narrow form. In 

 the case of the camellia and eucalyptus, the 

 lateral movement is considerable, and the 

 leaves of the cissus sometimes describe circular 

 arcs in portions of their revolution. The lines 

 are very irregular, and often broken by subor- 

 dinate loops and triangles. The leaves of some 

 plants describe several ellipses in the course of 

 a day. A large ellipse is sometimes formed on 

 one day and two small ones on the following. 

 A tendency toward nyctitropism is observed in 

 the leaves of many plants which do not possess 

 the special formation, the puhinus, by which 

 the sleep of leaves is accomplished. The peri- 

 odicity in the circumnutation of a great many 

 leaves, and their frequent habit of rising in the 

 evening and lowering in the morning, affords 

 a basis for the explanation of nyctitropism as 

 a specialized function in accordance \\iththe 

 theory of natural selection. Nyctitropism is 

 characteristic of a large number of angiosperms, 

 and of species belonging to several different 

 genera of monocotyledonous plants. It has 

 also been observed in the genus Abies of the 

 sub-class of gymnosperms, and in the family 

 Marsalacece of the acotyledons. The genera in 

 which the leaves are raised toward the zenith 

 at night and those in which they point down 

 toward the ground are about equal in number. 

 The cotyledons of a long list of plants exhibit 

 the same sleep-movements, and are usually pro- 

 vided with pulvini, like the leaves of nyctitro- 

 pic plants. The amount of angular deflection 

 from the horizon which should be considered 

 as characterizing nyctitropism is arbitrarily 

 fixed by Darwin at 60, at which angle the 

 exposure to the sky is just half as great as in a 

 horizontal position. The leaves of sixty-nine 

 genera have been proved to be nyctitropic. Of 

 these thirty-seven elevate and thirty-two sink 

 their leaves at night. In many plants the leaves 

 are raised or lowered through an angular space 

 of 90. The sleep of cotyledons, although 

 scarcely noticed before the observations of the 

 Messrs. Darwin, appears to be of more frequent 

 occurrence than the sleep of leaves. In the 

 large majority of cases the movement is a rising 

 one. There is no agreement or connection 

 between the behavior of the leaves and the 

 cotyledons of the same plant ; in a few species 

 the undeveloped leaflets act in a different man- 

 ner from the leaves; in some cases the leaves 

 of young plants sleep while those on full-grown 

 individuals do not. The leaves and cotyledons 

 of sleeping plants perform one revolution in 

 twenty-four hours. They are in continual mo- 

 tion, but this motion is considerably accelerated 



