62 PHYSIOLOGY [VEGETABLE. apices describing nearly a circle, a revolution taking from two to five minutes. A familiar example of an induced movement is afforded liy the leaves of the Sensitive Plant ( Mimosa pud ica). When a leaf is touched the lateral leaflets close in pairs, folding upwards and forwards, and, if the stimulus be strong enough, the main petiole sinks downwards. The movement of this plant is of special interest, inasmuch as it affords an instance of the transmission of a stimulus. It suffices, namely, to touch the terminal leaflet to cause the closing of the successive pairs of lateral leaflets and the sinking down of the main petiole. Another example of induced movement is afforded by the so-called "sleep" of plants; the leaves of many plants, namely, take up during the night a position different from that which they take up during the day. Taking the Sensitive Plant as an example, during the day its leaflets are widely expanded, and its main petioles are directed obliquely upwards ; at night its leaflets are folded together and its main petioles are directed obliquely downwards ; it takes up at night, in fact, a position similar to that which is induced by a touch. Excessive illumina tion tends to induce closing. Other examples are afforded by the mobile stamens of the Cynarcse and of Bcrbcris and Mahonia. The relation of thesa movements to external conditions is as follows : External 1. Temperature. These movements, .like the slow movement of condi- growth, only take place within certain limits of temperature, which, tions of however, vary in different cases. Movements of protoplasm, speak - these ing generally, will only go on at temperatures between and 50 C., move- and between these limits there is an optimum temperature at which ments. they are most rapid. In the case of movements of variation the lower limit lies considerably higher, from 15 to 20 C. The fore going illustrates the tonic relation between temperature and move ment. Sudden changes of temperature have a stimulating effect. For instance, Dutrochet observed that the protoplasm of the inter- nodal cells of a Chara exhibited rotation in water at 7 C., which soon ceased when the plant was placed in water at 32 C. ; after some time the movement returned, and was again arrested on replacing the plant in water at 7 C. 2. Light. In most cases of protoplasmic movements light ap pears to exert no influence ; in other cases it exerts a tonic influ ence. For instance, Engelmann has discovered a form of Bacterium, termed by him Bacterium jyhotomctricum, which is only mobile when exposed to light. Again, organs which exhibit spontaneous movements of variation, like the leaflets of Hcdysarum, or induced movements, like the leaflets of Mimosa, lose their power of move ment when kept in darkness for a day or two. Exposure to light, or, as it is termed, the state of "phototonns," is an essential condi tion of their movement. Bright light tends to arrest movement. For instance, the protrusion of pseudopodia by the plasmodium of ^Ethalium s .pticum is less active in light than in darkness. This is well shown in the sleep of plants, alluded to above. The "diurnal position " of the leaves is due to exposure to light, the " nocturnal position " to its absence. This is perhaps most clearly exhibited by the Sensitive Plant. Under the normal alternation of day and night the leaves assume alternately the diurnal and nocturnal positions. If a plant be kept for some time in darkness, at a suit able temperature, it will be found that the leaves exhibit periodic movements of opening and closing. They are, in fact, endowed, like those of the Telegraph Plant, with the power of spontaneous movement, which is arrested when the plant is exposed to light. Variations in the intensity of light act as stimuli. For instance, a sudden variation will cause the closing of the leaves of Mimosa. Light also exercises a directive influence on mobile protoplasm in some cases. For instance, when bright light falls obliquely on a plasmodium of dSthalium scpticum, it causes it to creep away from the light. Again, the chlorophyll-corpuscles in the cells of leaves exposed to bright light are found to accumulate on those surfac.es of the cells which are least exposed, to assume what is termed the position of " apostrophe," a change of position which is due to move ments in the protoplasm in which the corpuscles are embedded. Finally, the direction of movement of ciliated zoospores swimming in water is affected by light. When light falls obliquely upon a vessel of water containing zoospores, they place themselves so that their long axes are more or less nearly parallel to the direction of the incident rays, and it is along this line that they move. They may either move towards the incident light or away from it, the direction being apparently determined by the intensity of the light, by the age of the zoospores, and by the amount of oxygen in the water. Protoplasmic masses which respond to the directive action of light are said to be " photo tactic." 3. Other Stimuli. It has been mentioned that movements may be induced in the Sensitive Plant by mechanical stimulation, by variations of temperature, and variations in the intensity of light. They may also be induced by electrical and chemical stimuli. The effect of an electrical stimulus on protoplasm exhibiting the amoeboid movement is to cause retraction of the pseudopodia. It arrests also the rotating movement of the protoplasm for a time. 4. Oxygen. The presence of oxygen is an essential condition of movement of any kind, in the case at least of aerobiotic plants. It appears that anaerobiotic plants (Schizomycdes) are mobile in the absence of oxygen. Nature and Mechanism of Movements. On comparing the state- Condi ments which have been made above as to the movements of grow- tions ing organs and of mature mobile organs, their general similarity essenti is at once apparent. The spontaneous movement of growth is to mo comparable to the spontaneous movements of protoplasm and of meiit. mobile organs, and the performance of the former is dependent upon the same external conditions as the latter. The reaction to the influence of external agents is the same In many instances ; for example, strong light arrests growth, and it arrests also the spontaneous movements of the leaves of Mimosa and other plants, and contact stimulates tendrils as it stimulates the leaves of the Sensitive Plant. Again, light exercises a directive influence on the growth of growing organs ; it also exercises a directive influence on the movements of zoospores and plasmodia. These considerations lead to the conclusion that the causes of the movements must in all cases be the same. It has been already pointed out that growth and movement are expressions of the expenditure of energy on the part of the organism, that they are dependent upon the decomposition of some complex substance forming part of, or at least present in, the protoplasm. The conditions which are essential to movement of any kind are, then, these : that the decomposable substance in question is formed and decomposed in sufficient quantity, in other words, that the protoplasm is irritable ; that the protoplasm is capable of mani festing by a molecular change, which may be accompanied by a change in external form, the evolution of energy attending the decomposition ; and, finally, in the case of protoplasm surrounded by a cell-wall, that the anatomical structure is such as to permit of a movement ensuing upon the change in the protoplasm. It is clear that, if the cell-wall is rigid, no change in the protoplasm can cause a change in form of the cell as a whole. We may regard spontaneous movement as being due to the Spon- spontaneous decomposition of the decomposable substance whereby taneou the protoplasm undergoes a molecular change. The automatic move- decomposition not unfrequently takes place, as in the case of the nient. Telegraph Plant, at regular intervals, so that the movement is rhythmic or periodic. Spontaneous movement is most active when a certain favourable combination of external conditions is ensured ; any variation in the combination leads to a diminution in the activity, or even to complete arrest, of the movement. External conditions may affect the process either of formation or of decom position of the decomposable substance. For instance, movements are arrested at a low temperature, most probably because either the formation of the decomposable substance or the necessary explosive decomposition does not take place under such circumstances with sufficient activity. Again, when movement is arrested at a high temperature, or by continuous darkness, it is probably for similar reasons. In this way the tonic effect of external conditions may be accounted for. The stimulating effect of external agents may be accounted for in a similar manner. Movement, whether spontaneous or induced, is regarded as a phenomenon of contractility (see above, p. 13), but we have at present no knowledge of the exact nature of the molecular changes which constitute a contraction. It must be borne in mind not only that protoplasm contracts, but that, after contraction, it returns to its condition of rest. Spontaneous movement is the expression of automatic contraction. External tonic conditions either promote or retard movement, by cither promoting or retarding contraction and recovery. Stimuli induce contraction. The mechanism of the movements of protoplasm-masses appears Media to be as follows. Taking first the case of the ama-boid movement, ism of the protrusion of pseudopodia is due to a molecular change, of the move- nature of a contraction of the protoplasm, which takes place in the ments ectoplasm at the spot where the pseuolopodium is to be formed, an elevation being gradually produced into which the more fluid cndo- plasm is. as it were, sucked. The rotating movement of protoplasm appears to depend upon a kind of amceboid movement taking place constantly in one direction, to be, that is, a creeping movement. Ciliary movement appears to depend on the alternate contraction of each longitudinal half of the cilium. It is not possible at present to attempt any explanation of the directive influence of light on moving protoplasm, but the fact itself is of great physiological importance. The mechanism of the movements of organs, whether unicellular or multicellular, in which the cell -wall has to be considered is more complicated. The cells possess the structure described above : they consist of a cell -wall lined by the protoplasmic primordial utricle enclosing the cell-sap. They are, moreover, in a state of turgidity, that is, they are tensely filled Avith water. The state of turgidity in a cell depends upon three conditions, (1) upon a tendency to absorb more water in consequence of the presence of osmotic-ally active substances dissolved in the cell-sap ; (2) upon the resistance offered by the primordial utricle to the escape of water from the cell; (3) upon the elasticity of the cell -wall.
The elasticity of the primordial utricle is so small that it may be