With respect to the periodical movements of the cotyledons, those of several young seedlings formed together at 4 P.M. an angle of about 60o, and at 10 P.M. their lower parts stood vertically and were in contact; their tips, however, as is usual in the genus, were permanently reflexed. These cotyledons, at 7 A.M. on the following morning, were again well expanded.

Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbitaceae).--A seedling opened its cotyledons, the movements of which were alone observed, slightly on June 27th and closed them at night: next day, at noon (28th), they included an angle of 53o, and at 10 P.M. they were in close contact, so that each had risen 26 1/2o. At noon, on the 29th, they included an angle of 118o, and at 10 P.M. an angle of 54o, so each had risen 32o. On the following day they were still more open, and the nocturnal rise was greater, but the angles were not measured. Two other seedlings were observed, and behaved during three days in a closely similar manner. The cotyledons, therefore, [page 43] open more and more on each succeeding day, and rise each night about 30o; consequently during the first two nights of their life they stand vertically and come into contact.

Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1 inch in length, apex only 4 3/4 inches from the vertical glass, on which its movements were traced from 7.35 A.M. July 11th to 9.5 A.M. on the 14th. Figure here given reduced to one-third of original scale.

In order to ascertain more accurately the nature of these movements, the hypocotyl of a seedling, with its cotyledons well expanded, was secured to a little stick, and a filament with triangles of paper was affixed to one of the cotyledons. The observations were made under a rather dim skylight, and the temperature during the whole time was between 17 1/2o to 18o C. (63o to 65o F.). Had the temperature been higher and the light brighter, the movements would probably have been greater. On July 11th (see Fig. 30), the cotyledon fell from 7.35 A.M. till 10 A.M.; it then rose (rapidly after 4 P.M.) till it stood quite vertically at 8.40 P.M. During the early morning of the next day (12th) it fell, and continued to fall till 8 A.M., after which hour it rose, then fell, and again rose, so that by 10.35 P.M. it stood much higher than it did in the morning, but was not vertical as on the preceding night. During the following early morning and whole day (13th) it fell and circumnutated, but had not risen when observed late in the evening; and this was probably due to the deficiency of heat or light, or of both. We thus see that the cotyledons became more widely open at noon on each succeeding day; and that they rose considerably each night, though not acquiring a vertical position, except during the first two nights.

Cucumis dudaim (Cucurbitaceae).--Two seedlings had opened [page 44] their cotyledons for the first time during the day,--one to the extent of 90o and the other rather more; they remained in nearly the same position until 10.40 P.M.; but by 7 A.M. on the following morning the one which had been previously open to the extent of 90o had its cotyledons vertical and completely shut; the other seedling had them nearly shut. Later in the morning they opened in the ordinary manner. It appears therefore that the cotyledons of this plant close and open at somewhat different periods from those of the foregoing species of the allied genera of Cucurbita and Lagenaria.

Fig. 31. Opuntia basilaris: conjoint circumnutation of hypocotyl and cotyledon; filament fixed longitudinally to cotyledon, and movement traced during 66 h. on horizontal glass. Movement of the terminal bead magnified about 30 times, here reduced to one-third scale. Seedling kept in hot-house, feebly illuminated from above.

Opuntia basilaris (Cacteae).--A seedling was carefully observed, because, considering its appearance and the nature of the mature plant, it seemed very unlikely that either the hypocotyl or cotyledons would circumnutate to an appreciable extent.

Charles Darwin

All Pages of This Book