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Show 12 CIRCUMNUTA'l'ION OF SEEDLINGS. CHAP. I. 2nd in the same general direction, and in a similar zigzag manner. From the radicle not being quite perpendicular when the filament was affixed geotropism came into play at once; but the irregular zigzag course shows that there was growth (probably preceded by turgescence), sometimes on one and sometimes on another side. Occasionally the bead remained stationary for about an hour, and then probably growth occurred on the side opposite to that which caused the g~otropic curvature. In the case previously described the basal part of the very short radicle from being turned vertically upwards, was at first very little affected by geotropism. Filaments were affixed in two other instances to rather longer radicles protruding obliquely from seeds which had been turned upside down; and in these cases the lines traced on the horizontal glasses were only slightly zigzag, and the movement was always in the same general direction, through the action of geotropism. All these observations are liable to several causes of error, but we believe, from what will hereafter be shown with respect to the movements of the radicles of other plants, that they may be largely trusted. Bypocotyl.-The hypocotyl protrudes through the seed-coats as a rectangular projection, which grows rapidly into an arch like the letter U turned upside down .0.; the cotyledons being still enclosed within the seed. In whatever position the seed may be embedded in the earth or otherwise fixed, both legs of the arch bend upwards through apogeotropism, and thus rise vertically above the ground. As soon as this has taken place, or even earlier, the inner or concave surface of the arch grows more quickly than the upper or convex surface; and this tends to separate the two legs and aids in drawing the cotyledons out of the buried seed-coats. By the growth of the whole arch the cotyledons are ultimately dragged from beneath the ground, even from a considerable depth; and now the hypocotyl quickly straightens itself by the increased growth of the concave side. Even whilst the arched or doubled hypocotyl is still beneath the ground, it circumnutates as much as the pressure of the surrounding soil will permit; but this was difficult to observe, because as soon as the arch is freed from lateral pressure the two legs begin to separate, even at a very early age, before the arch would naturally have reached the smface. Seeds were allowed to germinate on the surface of damp earth, and after they had fixed themselves by their radicles, and after the, as yet, only CHAP. I. BRASSIOA. 13 slightly arched hypocotyl had become nearly vertical, a glass filament was affixed on two occasions ncar to the base of the basal leg (i.e. the one in connection with the radicle), and its movements were traced in darkness on a horizontal glass. The result was that long lines were formed running in nearly the plane of the vertical arch, due to the early separation of the two l~gs now freed from pressure; but as the lines were zigzag, showmg lateral movement, the arch must have been circum~ utating, whilst it was straightening itself by growth along its mner or concave surface. A somewhat different method of observation was next followed: Fig. 3. -------· -----~-------- -----------------------\ Bt·assicrt oleracea: circumnutatincr movement of buried and arched h t I (d. I 'll · " ' ypo- co }' . 1m y 1 ummated from abo,re), traced on horizontal glass during 45 hours. Movement of beau of filament maO"nified about 25 t' . and here reduced to one-half of original scale. 0 lmes, as soon as the earth with seeds in a pot began to crack the surface was removed in parts to the depth of ·2 inch; a~d a filamen.t was fixed to the basal leg of a buried ~.!ld arched hypocotyl, J~Ist above the summit of the radicle. The cotyledons were still almost completely enclosed within the much-cracked se~d-coats; and these were again covered up with damp adhesive SOil pressed pretty firmly down. The movement of the filament was traced (Fig. 3) from 11 A.M. Feb. 5th till 8 A.M. Feb. 7th. By this latter period the cotyledons had been dragged from beneath the. pressed-down earth, but the upper part of the hypocotyl still formed nearly a right anglo with the lower part. The tracing shows that the arched hypocotyl tends at this early |
