RHYTHMIC MOVEMENTS IN PLANTS AND ANIMALS 249 



of the day and night, and a floral clock can be arranged to indicate the hours with nearly as much certainty as the 

 shadow on the sun-dial. 



A hving clock composed of plants, or parts of plants, opening or closing at particular intervals, is only 

 possible in hving structures. Light, heat, cold, moisture, dryness, &c., are not necessary to the opening and closing 

 movements. 



This is proved by the fact that parts of plants open and close at different periods of the day and night ; the 

 stomata of the leaves, as stated, opening in damp weather, when we would naturally expect them to close, and 

 closing in dry weather, when we would naturally expect them to open. 



The Volvox globator, which is an aquatic plant, opens and closes its vacuoles at stated intervals, as apart from 

 every form of external stimulation. Other plants, when in a vigorous, healthy condition, exhibit rhythmical move- 

 ments, and notably the Hedysarum (Desmodium gyrans), a native of the East Indies. The leaf in this plant is 

 unequally pinnate, consisting of a larger leaflet at the end of the stalk, and two pairs of leaflets placed laterally. The 

 smaller leaflets come towards and recede from each other with a jerking motion, every three minutes or so. The 

 movements of the heart are certainly not more regular than those of the leaves now referred to. It cannot be the 

 hght or heat which produces the movements of the leaflets, for they go on in the dark in a reduced temperature ; 

 and as they are most regular when the plant is most healthy, we are not entitled to assume inherent irritability or 

 extraneous stimulation. The movements in the leaflets in some senses foreshadow a heart, and are no doubt con- 

 nected with the nutrition of the plant. They teach us one very important fact, namely, that living organs and 

 portions thereof can come and go, contract, expand, and perform stated motions at stated intervals, without the 

 presence of nerves, muscles, elastic and other tissues, unless, perchance, these exist in an undifferentiated form, 

 which is by no means improbable. According to Fee, the fluids drawn to the surface of a plant during hght are 

 kept in equiUbrium by rhythmical evaporation ; the rhythmical movements of the leaves being referable to vital 

 changes in the cell contents and vessels. 



As a proof that the presence or absence of moisture will not account for all the phenomena witnessed in cells 

 and other structures, it may be stated that in the sensitive plant {Mimosa pudica) there is a swelling at the base 

 of the petiole, the cells of which constitute, as it were, two springs which act in opposite directions ; so that if from 

 any cause the one be paralysed, the other pushes the leaf in the direction of least resistance. The springs, if they 

 may be so called, situated at the base of the petiole are set in motion by the rush of fluid, creating a turgid state 

 of the one set of cells, and an empty state of the other. A kind of rhythmic movement is thus produced. What 

 is it, one naturally inquires, which gorges the one set of ceUs and empties the other, if it be not a vital power exer- 

 cised by the plant 1 The fluid is present to both sets of cells alike. The same fluid certainly cannot stimulate the 

 one set of cells to contract or shorten and the other to enlarge or lengthen ; and besides, so far as known, there are 

 no contractile tissues present in the plant. Neither can the presence of moisture act as an irritant, moisture being 

 necessary to the life of the plant, and a normal part of it. The only explanation that can be given is, that the plant 

 lives, and that it sucks in moisture by the one set of cells, and ejects moisture by the other set, just as one part of 

 the heart sucks in blood while another expels it. 



RHYTHMIC MOVEMENTS IN PLANTS AND ANIMALS 



§ 50. These are Repetitions of Rhythms occurring in External Nature. 



It may, at first sight, seem far-fetched to say that the rhythms in plants and animals are the outcome of 

 rhythms occurring in the inorganic kingdom. As, however, the organic kingdom obtains all its matter, and no 

 inconsiderable portion of its force, from the inorganic kingdom, it need occasion no surprise if rhythms, of a very 

 distinctive and well-marked character, make their appearance in plants and animals. 



The term rhythm is derived from the Greek pvO/ixoi. Strictly speaking, it means a measured motion. 

 In a wider sense, it is a dividing into short portions by a regular succession of motions, impulses, sounds, accents, 

 &c., as applied to music, poetry, the dance, &c. In its widest sense, it may be regarded as an advancing and 

 receding, a going and coming, a giving and taking, an opening and closing, a lengthening and shortening, &o. ; 

 the repetition at intervals, and the time and manner of the repetition, being accessory to the main idea. 



WHle the element of time enters into our conception of all rhythms, it is necessary to explain that in physiology 

 the time-limit for the recurrence of the movement forming the rhythms varies very considerably. Thus m the 

 vermicular movements of the intestine during digestion, the movements proceed with great regularity and rapidity— 

 so much so, that at times the rhythms may be said to run into each other : in the CBSophagus m swallowing, and m 

 the stomach in discharging chyme, they are considerably slower, 



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VOL. I. 



