OCR Text |
Show 108 CIRCUMNUTA'fiON OF HYPOCOTYLS, ETC. CHAP. II. the extreme amount of movement from side to side of their circumnutating stems was small; that of the hypocotyl of Githago segetum was about ·2 of au inch, and that of Cucurbita ·ovifera about ·28. A very young shoot of Lathyrus nissolia moved about ·14, that of an American oak ·2, that of tho common nut only ·04, and a rather tall shoot of the Asparagus ·11 of an inch. The extreme amount of movement of the sheath-like cotyledon of Phalaris Canariensis was ·3 of an inch; but it did not move very quickly, the tip crossing on one occasion five divisions of the micrometer, that is, r~0th of an inch, in 22 m. 5 s. A seedling Nolana prostrata travelled the same distance in 10 m. 38 s. Seedling cabbages circumutate<l much more quickly, for the tip of a cotyledon crossed 1l 0th of an inch on the micrometer in 3 m. 20 s. ; ai}d this rapid movement, accompanied by incessant oscillations, was a wonderful spectacle when beheld under the microscope. The absence of light, for at least a . day, <loes not interfere in the least with the circumnutation of the hypocotyls, epicotyls, or young shoots of the various dicotyledonous seedlings observed by us; nor with that of the young shoots of some monocotyledons. rl'he circumnutation was indeed much plainer in darkness than in light, for if the light was at all lateral the stem bent towards it in a more or less zigzag course. Finally, the hypocotyls of many seedlings are drawn during the winter into the ground, or even beneath it so that they disappear. This remarkable process, which apparently serves for their protection, has been fully described by De Vries.* He shows that * 'Bot. Zeitung,' 1879, p. 649. See also W inkier in ' V erhandl. des Bot. Vereins der P. Brlmden-burg,' Jahrg. xvi. p. 16, ~s guoted by Haberlandt, ' Schutzomrwbun· gen der Keimpflanze,' 1877, p. 52. CHAr. II. CIRCUMNUTATION OF COTYLEDONS. 10D it is effected by the contraction of the parenchymacells of the root. But the hypocotyl itself in some cases contracts greatly, and although at first smooth becomes covered with zigzag ridges, as we observed with Githago seget·um. How much of the drawing down and burying of the h ypocoty l of Opunt,ia basilaris was due to the contraction of this part anJ how much to that of the radicle, we did not observe. Circumnutation of Cotyledons.-- vVith all the dicotyledonous seedlings described in the last chapter, the cotyledons were in constant movement, chiefly in avertical plane, anti commonly once up and once clown in ,the course of the 2L! hours. But there wore many excep-tions to such simplicity of movement; thus the cuty ledons of Ipoma3a cmrulea moved 13 times either upwards or downwards in the course of 16 h. 18m. 1'hose of Orealis rosea moved in the same manner 7 times in the course of 24 h.; and those of Cassia tora described 5 irregular ellipses in 9 h. The coty lodons of some individuals of Mimosa pudica and of Lot~ts J acobmus moved only once up and down in 2:1: h., whilst those of others performed within the same period an additional Sl~HI,ll oscillation. 1'hus with different species, and w1th different ind.ividnals of the same species, there were many gradations from a single diurnal movement to oscillations as complex as those of the Ipomooa and Cassia. The opposite cotyledons on the same seedling move to a certain extent inuependeutly of o~o another. 'l'his was conspicuous with those of Oxal~s sensitiva, in which one cotyledon mio·ht be d . b see~ urmg the daytime rising up until it stood vertwally, whilst the opposite one was sinking down. Alt~ough the movements of cotyledons were generally m nearly the same vertical plane, yet their upward and d.ownward courses never exactly coin- : l!fl t• ...r ,.. ;., .:)r |