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Show 46 CIRCU1\1NU'rA'riON OF SEEDLINGS. CHAl' . l. on the followino· day with nearly the same result; and thoro can be no doubt about the circumnutation of tho bypocotyl. Fig. 33. ,,, ......... ' .. - --------:.-------------- _/_/······'/ P1·im11la Sinensis : conjoint circurnnutation of hypocotyl no el cotylctlon, traced on vertical glass, from 8.40 A.M. to 10.45 P.M. lVIoYem c nt~ of bead magnified about 26 times. Cyclamen Perwicnm (Primuluceoo).-This plant is generally supposed to produce only a single cotyledon, but Dr. H. Grossnor * has shown that a second one is developed after a long interval of time. The bypocotyl is converted into a globular corm, even before the first cotyledon has broken through the ground with its blade closely enfolded and with its petiole in the form of an arch, like the arched hypocotyl or epicotyl of any ordinary dicotyledonous plant. A glass filament was affixed to a cotyledon, ·55 of an inch in height, tho petiole of which had straightened itself and stood nearly vertical, but with the blade not as yet fully expanded. Its movements were traced during 24} h. on a St,rpelirt sarpedon: circumnutatiou of hypocotyl, illuminated from 3bove, traced on horizontal glass, from 6.45 .A.M. June 26th to 8.45 A.M. 28th. Temp. 23°-24° C. ~ovement of bead magnified 21 t1mes. horizontal glliss, magnified 50 times; and in this interval it · described twq irregular ::;mall circles; it therefore circumnntates, though on au extremely small scale. Staptlia sa1·pedun (Asclepiadeoo). - This plant, when mature, resembles a cactus. The flattened hypocotyl is fleshy, enlarged in the upper part, and bears two rudimen-tary Qoty ledons. It brcakH through the ground in an arched form, with the rudimentary cotyledons closed or in contact. A filament was affixed almost * 'Bot. Zeitung,' 1874, p. 837. CrrAP. I. IPOl\1mA. 47 vertically to the hypocoty I of a seedling half an inch high; and its movements were traced during 50 h. on a horizontal gll1ss (Fig. 34). From some unknown cause it bowed itself to one side, and us this was effected by a zigzag course, it probably circumnututed; but with hardly any other seedling observecl by us was this movement so obscurely shown. Jpomrea ccerulea vel Pharbitis n·il (Couvolvulacooo).-Seedlings of this plant were observed because it is a twiner, the upper internodes of which circumuutate conspicuously; but, like other twining plants, the first few internodes which rise above the ground are stiff enongh to support themselves, and therefore do not circumnutate in any plainly recognisable manner.* In this particular instance tho fifth internode (including the hypocotyl) was the first which plainly circumnutated and twined round a stick. We therefore wishecl to learn whether circumnutation could be observed in the hypocotyl if carefully observed in our usual manner. Two seedlingR were kept in the dark with filaments fixed to tho upper part of their hypocotyls; but from circumstances not worth explaining their movements wore traced for only a short time. One moved thrice forwards and twice backwards in nearly opposite directions, in the course of 3 h. 15m. ; and the other twice forwards and twice backwards in 2 h. 22m. The hypocotyl therefore circumuutated at a remarkably rapid rate. It may here be added th:.tt a filament was affixed transversely to the summit of the second internode above the cotyledons of a little plant 3t inches in height; and its movements were traced on a horizontal glass. It circumuutated, and the actual distance travelled from side to side was a quarter of an inch, which was too small an amount to be perceived without the aid of marks. The movements of tho cotyledons are interesting from their complexity and rapidity, :.tnd in some other respects. Tho hypocotyl (2 inches high) of a vigorous seedling was secured to a stick, and a filament with triangles of paper was affixed to ono of the cotyledons. The plant was kept all day in tho hot-house, and at 4.20 P.M. (June 20th) was placed under a skylight in the house, and observed occasionally during tho evening and night. It fell in a slightly zigzag line to a moderate extent from 4.20 P.M. till 10.15 P.M. When looked at shortly aftel' midnight (12.30 PM.) it had risen a very little, and considembly by * 'Movements and Habits of Climbing Plants,' p. 3:3, 1875. |