The quality of recurring at intervals

Usage examples of periodicity.

Distinction between heliotropism and the effects of light on the periodicity of the movements of leaves--Heliotropic movements of Beta, Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements of tendrils of Bignonia--Of flowerpeduncles of Cyclamen--Burying of the pods--Heliotropism and apheliotropism modified forms of circumnutation--Steps by which one movement is converted into the other Transversalheliotropismus or diaheliotropism influenced by epinasty, the weight of the part and apogeotropism--Apogeotropism overcome during the middle of the day by diaheliotropism--Effects of the weight of the blades of cotyledons--So called diurnal sleep--Chlorophyll injured by intense light--Movements to avoid intense light.

Cases of semimonthly menstruation and many similar anomalies of periodicity are spoken of.

There are many cases of constipation on record lasting longer than this, but none with the same periodicity and without change in the excrement.

It was these rhythms, the very notion of periodicity, that were the basis of religion: such patterns were the expression of mysterious forces.

This upward movement differs from one of the great diurnal oscillations above described only by the position being permanent during the night and by its periodicity, as it always commences late in the evening.

The regular periodicity of these movements is destroyed, as we shall hereafter see, if the seedlings are kept in the dark.

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

Darkness, therefore, during a day and a half does not interfere with the periodicity of their movements.

It has also been shown by us, that although leaves subjected to darkness for a moderately long time continue to circumnutate, yet the periodicity of their movements is soon greatly disturbed, or quite annulled.

As the leaves of most plants assume their proper diurnal position in the morning, although light be excluded, and as the leaves of some plants continue to move in the normal manner in darkness during at least a whole day, we may conclude that the periodicity of their movements is to a certain extent inherited.

But there is frequently a distinct trace of periodicity in the circumnutating movements of nonsleeping leaves and cotyledons.

Professor Perrey, of Dijon, France, is endeavoring to prove that there is a periodicity in earthquakes, synchronous with that in the tides of the ocean, the greatest number occurring at the time of new and full moon.

That idea never washed with me, since to get the 26-million-year periodicity you had to use the late Ordovician dyings, which were obviously just the result of plate tectonics moving the supercontinent Gondwanaland over the south pole, causing an ice age.

The regular periodicity of these movements is destroyed, as we shall hereafter see, if the seedlings are kept in the dark.

But there is frequently a distinct trace of periodicity in the circumnutating movements of nonsleeping leaves and cotyledons.