CIRCUMNUTATION. 



109 



the arch straightens out and the stem becomes 

 erect. This curving of the stems is of the 

 nature of epinasty or hyponastj. These arched 

 organs are continually circumnutating or en- 

 deavoring to circumnutate while working up 

 through the ground, and this movement prob- 

 ably assists them to break through. The part 

 of a seedling which first issues forth is always 

 strongly acted upon by apogeotropism. What- 

 ever portion of the arch first emerges from the 

 ssed coat immediately turns upward, and the 

 arch assumes a vertical position as rapidly as 

 the superincumbent earth will permit. The 

 tendency of_the hypocotyi to take an arched 

 form is widespread, and occurs sometimes in 

 plants whose cotyledons are hypogean, and 

 where it can be of no possible service. The 

 hypocotjds and the upper portion of the radi- 

 cles of beans were observed to curve them- 

 selves when allowed to germinate in a revolv- 

 ing vessel by which the force of geotropism 

 was neutralized until they were deflected by an 

 average angle of 63. This phenomenon was 

 first remarked by Sachs, and is called by Dar- 

 win Sachs's curvature. As soon as the first 

 ray of light touches the pushing shoot the 

 potent force of heliotropism comes into play. 

 The upper surface of the hypocotyi is probabfy 

 sensitive to light in the case of dicotyledons, 

 and in monocotyledons, which do not emerge 

 from the ground with bowed stems, in the tips 

 of the cotyledons as they protrude. In the 

 grasses the cotyledon is furnished with a sharp, 

 hard crest, which serves for breaking through 

 the soil. 



The circumnutation of the arched hypocotyls 

 was a puzzling phenomenon until it was ex- 

 plained by the undulatory nutation of inter- 

 nodes observed by Wiesner in certain seedlings 

 whose tips are bent downwar.l. In such bsnt 

 steins the turgescence and increased growth 

 do not occur simultaneously along the whole 

 of the outer side or of the inner side of the 

 bowed stem. If they did, there would evidently 

 be no circumnutation. The spasmodic growth 

 occurs on one side of one leg of the arch and 

 on the opposite side of the other leg, while at 

 the elbow there is a zone where the growth is 

 equal on all sides. Before the cotyledons ex- 

 pand, the arched hypocotyi usually straightens 

 itself by increased growth on the concave side, 

 reversing the process by which it first became 

 bent, until ultimately no trace of the curvature 

 remains. The erection of the stem is accom- 

 panied with a revolving movement, and is 

 therefore a modification of circumnutation. 

 The hypocotyls and plumules of seedlings after 

 straightening circumnutate constantly, describ- 

 ing oval figures which vary greatly in size in dif- 

 ferent plants, the period of the revolution also 

 varying considerably. The extreme amount of 

 movement from side to side is relatively very 

 great in the hypocotyls, amounting to "28 inch 

 in the egg-squash, -11 inch in a tali shoot of 

 asparagus, -2 inch in the American oak, and 

 only -04 inch in the European walnut. The 



tip of the cotyledon of a seedling cabbage was 

 observed under the microscope to move very 

 rapidly, traveling -01 inch in a little more than 

 three minutes, the forward movement being 

 accompanied by incessant oscillations. The leaf 

 of the dionsea circumnutates in the same vi- 

 bratory manner. A great number of cells must 

 distend at the same time to produce this jerky 

 movement ; but it is not known whether the 

 harmonious turgescence, is of an intermittent 

 character, or whether, as they become more and 

 more turgescent, they suddenly break the strain 

 of the opposing tissues and cause the whole 

 part to yield. 



The stem of the seedling circumnutates at a 

 comparatively rapid rate. In some plants, such 

 as the egg-squash and cabbage, four elliptic 

 revolutions were performed in twelve hours by 

 the hypocotyi, in some others only one. The 

 ellipses described are very narrow in some 

 plants, and in others, for example the oak, they 

 approach a circle. They are frequently com- 

 plicated by minute loops and zigzag lines. The 

 cotyledons of all dicotyledonous plants circum- 

 nutate, moving upward and downward gener- 

 ally. The period of circumnutation is com- 

 monly twenty-four hours, though in some cases 

 many revolutions are accomplished in that 

 time. The two cotyledons circumnutate to a 

 great extent independently of each other. In 

 a great majority of cases the circumnutating 

 movement was nyctitropic, or of the nature of 

 the sleep of leaves. Most hypocotyls and coty- 

 ledons are extremely sensitive to light. Coty- 

 ledons are paratonically heliotropic, that is, 

 their daily periodic movements are greatly and 

 quickly disturbed by the sudden admission or se- 

 clusion of light, or by variations in its intensity. 



Seedling plants are exceedingly heliotropic 

 as a rule. The sensitiveness to light seems to 

 reside in the tip of the cotyledon, which trans- 

 mits the bending movement for some distance 

 down the stem. The basal portion of the stem 

 after a certain height has been attained does 

 not bend. Tho effects of light upon seedlings 

 are exceedingly variable upon different individ- 

 uals of a species. Cotyledons are sensitive to 

 differences in the illumination on opposite sides 

 which are too slight to be perceived by the 

 human eye. Light seems to act upon the tis- 

 sues of plants in a similar manner to what it 

 does on the nervous system of animals. Coty- 

 ledons which were exposed to light for only a 

 couple of minutes at a time, and after long in- 

 tervals, bent toward the source of the light ; 

 and cotyledons which had been exposed to 

 sunlight, and others which had stood in the 

 shade on being placed together under an in- 

 termediate degree of illumination, moved in 

 directly opposite directions. The fact that the 

 sensitiveness of the cotyledons of young seed- 

 lings to light is confined to the tip, was proved 

 by covering the tip with tin-foil, or painted 

 gold-beater's skin, in which cases the stems 

 remained erect after exposure to a side-light, 

 although when the tips were covered with un- 



