Page:Popular Science Monthly Volume 18.djvu/519

Rh or ·001 of an inch, and then a slow retreat for part of the distance. No such movement could be detected with a two-inch object-glass, in the case of Drosera, and how far it is general is not known. Mr. Darwin says: "The whole hypocotyl (stem of a cotyledon) of a cabbage or the whole leaf of a Dionæa could not jerk forward unless a very large number of cells on one side were simultaneously affected. Are we to suppose that these cells steadily become more and more turgescent on one side, until the part suddenly yields and bends, inducing what may be called a microscopically minute earthquake in the plant; or do the cells on one side suddenly become turgescent in an intermittent manner—each forward movement thus caused being opposed by the elasticity of the tissues?"

Mr. Darwin has shown the importance of this ever-present movement in successive chapters upon modified circumnutation. By this phrase he means that pressure and other irritants, light and gravitation, do not directly cause movement; they only modify the spontaneous changes in the turgescence of the cells, which are always in progress, and of which circumnutation is a universal consequence. He thinks that, in the case of seedlings, ordinary or unmodified circumnutation is clearly of service, directly or indirectly; but, in the later stages of growth, it is from various modifications of this constant motion that the plant derives benefit.

More than half the volume is given to the modifications of circumnutation by epinasty and hyponasty; by nyctitropic or sleep-movements; and by the influence of light and of gravitation: but we can only glance at one of these, which is popularly styled the sleep of leaves, although there is probably no real analogy between the sleep of animals and that of plants.

 I. Seedling viewed obliquely from above, during the day: Rc, right cotyledon; Lc, left cotyledon; F, first true leaf. II. A rather younger seedling, viewed at night: Rc, right cotyledon raised, but its position not otherwise changed; Lc, left cotyledon raised and laterally twisted; F, first leaf raised and twisted so as to face the left twisted cotyledon. III. Same seedling viewed at night from the opposite side. The back of the first leaf, F, is here shown instead of the front, as in II.

Nyctitropic (night-turning) is the word used by Darwin to describe the sleep of leaves and occasionally of flowers, but as flowers are affected chiefly by changes of temperature instead of light, their