What is "alternations"

Usage examples of "alternations".

That the periodicity is determined by the daily alternations of light and darkness there can hardly be a doubt, as will hereafter be shown.

Conditions necessary for these movements--List of Genera and Families, which include sleeping plants--Description of the movements in the several Genera--Oxalis: leaflets folded at night--Averrhoa: rapid movements of the leaflets--Porlieria: leaflets close when plant kept very dry--Tropaeolum: leaves do not sleep unless well illuminated during day--Lupinus: various modes of sleeping--Melilotus: singular movements of terminal leaflet--Trifolium--Desmodium: rudimentary lateral leaflets, movements of, not developed on young plants, state of their pulvini--Cassia: complex movements of the leaflets--Bauhinia: leaves folded at night--Mimosa pudica: compounded movements of leaves, effect of darkness--Mimosa albida, reduced leaflets of--Schrankia: downward movement of the pinnae--Marsilea: the only cryptogam known to sleep--Concluding remarks and summary--Nyctitropism consists of modified circumnutation, regulated by the alternations of light and darkness--Shape of first true leaves.

In the second subclass the modification depends to a large extent on external agencies, such as the daily alternations of light and darkness, or light alone, temperature, or the attraction of gravity.

The circumnutating movement is thus at least partially periodic, no doubt in connection, as we shall hereafter see, with the daily alternations of light and darkness.

Finally, it will be shown that these movements result from circumnutation, much modified and regulated by the alternations of day and night, or light and darkness.

With respect to the periodicity of the movements of sleeping leaves, Pfeffer* has so clearly shown that this depends on the daily alternations of light and darkness, that nothing farther need be said on this * 'Die Periodischen Bewegungen der Blattorgane,' 1875, p.

The alternations of light and darkness merely give notice to the leaves that the period has arrived for them to move in a certain manner.

From the facts and considerations now advanced we may conclude that nyctitropism, or the sleep of leaves [page 413] and cotyledons, is merely a modification of their ordinary circumnutating movement, regulated in its period and amplitude by the alternations of light and darkness.

These movements have been ably investigated by Pfeffer, who has shown (as was first observed by Hofmeister) that they are caused or regulated more by temperature than by the alternations of light and darkness.

These movements, which are sometimes extremely complex, are regulated, though not directly caused, by the alternations of light and darkness.

It must be the result of some periodical change in the conditions to which they are subjected, and there can hardly be a doubt that this is the daily alternations of light and darkness.

The socalled sleep or nyctitropic movements of leaves are determined by the daily alternations of light and darkness.

Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as in the case of climbing plants, and secondly by its being rendered periodic in relation to the alternations of day and night.

When we speak of modified circumnutation we mean that light, or the alternations of light and darkness, gravitation, slight pressure or other irritants, and certain innate or constitutional states of the plant, do not directly cause the movement.

Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate on geographical distribution.