ELEMENTARY VITAL PHENOMENA 227 



circumference will become smaller (Fig. 90). But what is more 

 important for the present purpose is the diminution in size of the 

 whole cell with decrease of the turgor, for the tension of the elastic 

 cellulose coat will be decreased to the same extent as that of the 

 primordial utricle, and, as a result of its elasticity, the wall will 

 assume finally a circumference corresponding to its decreased 

 tension (Fig. 90, B, C, D). 



In the movements of plants now under consideration a change 

 of turgor takes place solely by the contraction of the primordial 

 utricle of certain cells for some cause, either spontaneously or as 

 the result of stimulation, in such a manner that water is squeezed 

 out of the cells ; the phenomenon passes away after some time, 

 and the turgor again appears pari passu with the disappearance of 

 the contraction. There thus appears under certain circumstances 

 a sudden diminution of the turgor and with it a diminution in 

 the size of the cell, and only gradually does the previous condition 

 return. 



In order that upon this principle a microscopic movement may 

 take place in a plant, the cells that undergo the change of 

 turgescence must have a definite arrangement. If in one of two 

 parallel rows of cells the turgor is suddenly diminished, so that the 

 cells become smaller, while in the other it remains unchanged, the 

 first row must shorten. Hence, according to simple mechanical 

 principles, a bending will occur with the concavity upon the 

 shortened side. At the same time the other side will be 

 extended passively. If, later, a gradual increase of turgor and a 

 lengthening of the cells upon the shortened side takes place, the 

 elasticity of the other side will assist the extension. 



Such a diminution of turgescence appears in many plants, 

 often very suddenly, both spontaneously and after mechanical 

 stimulation, and the result is a sudden movement of certain 

 parts. In most cases both the arrangement and the shape of the 

 cells that cause the movement are very complicated. As a 

 rule, at the base of the motile leaves, or petioles, small enlarge- 

 ments, called pulvini, are developed, the cells of which can 

 diminish their turgor very rapidly. One of the best-known 

 examples of this kind is the movement of the petioles in the sensi- 

 tive Mimosa pudica, which in the " waking " state, i.e., during the 

 day, are upright with the leaflets extended (Fig. 91, /, A, and 

 //, A), while in the " sleeping " state, i.e., at night, they are 

 depressed and the leaflets are folded upward together (Fig. 91,7,5, 

 and //, B). If a Mimosa- in the waking state be vigorously 

 shaken, the night position is suddenly assumed in the daytime. 



Upon the same principle depend numerous other movements of 

 the sensitive plants, such as those of the leaves of clover, the 

 stamens of barberry, the insect-catching organs of carnivorous 

 plants, and many others. 



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