MODIFIED CIRCUMNUT ATION: NYCTITROPIC OR
Innate or constitutional changes, independently of any external agency, often modify the circumnutating movements at particular periods of the life of the plant. The first change is the protrusion of the radicle, which begins at once to circumnutate. This movement is immediately modified by the attraction of gravity and rendered geotropic. With dicotyledonous seeds, after the protrusion of the radicle, the hypocotyl breaks through the seed-coats; but if the cotyledons are hypogean, it is the epicotyl which breaks forth. As the arching occurred in whatever position the seeds were placed, it is no doubt due to temporarily increased growth of the nature of epinasty or hyponasty along one side of the part. Although the nyctitropic movement of leaves and cotyledons are wonderfully diversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at night, that its upper surface is exposed as little as possible to full radiation.