(iv.) The movements of many Protozoa are affected greatly by Light. These movements have been distinguished into "photopathic," i.e. to or from the position of greatest luminosity; and "phototactic," along the direct path of the rays.[[30]] Those Protozoa that contain a portion of their cytoplasm, known as a "plastid" or "chromatophore" (see pp. [36], [39]), coloured by a green or yellow pigment are usually "phototactic." They mostly have at the anterior end a red pigment spot, which serves as an organ of sight, and is known as an "eye-spot." In diffused light of low intensity they do not exhibit this reaction, but in bright sunlight they rise to the surface and form there a green or yellow scum.
Most of the colourless Protista are negatively phototactic or photopathic; but those which are parasitic on the coloured ones are positively phototactic, like their hosts.
Here, as in the case of other stimuli,[[31]] the absolute intensity of the light is of importance; for as it increases from a low degree, different organisms in turn cease to be stimulated, and then are repelled instead of being attracted. The most active part of the spectrum in determining reactions of movement are the violet and blue rays of wave-length between 40 µ/10 and 49 µ/10, while the warmer and less refractive half of the spectrum is inert save in so far as it determines changes in the temperature of the medium.
(v.) The movements of many Protozoa are rendered sluggish by cold, and active by a rise of Temperature up to what we may term the "optimum"; the species becomes sluggish again as the temperature continues to rise to a certain point when the movements are arrested, and the being is said to be in a state of "heat-rigor." Most Protozoa, again, tend to move in an unequally heated medium to the position nearest to their respective optimum temperature. This is called "thermotaxy." The temperature to which Amoeba is thermotactic is recorded as 35° C. (95° F.); that of Paramecium is 28° C. (82° F.).
(vi.) Most active Protozoa tend to take up a definite position in respect to a current of Electricity passing through the medium, and in the majority of cases, including most Ciliates, Amoeba, and Trachelomonas, they orient their long diameters in the direction of the lines of force and swim along these to assemble behind the cathode. The phenomenon is called "galvanotaxy," and this particular form is "negative." Opalina (Fig. 41, p. [123]), however, and most Flagellates are "positively galvanotactic," and move towards the anode. H. H. Dale[[32]] has shown that the phenomenon may be possibly in reality a case of chemiotaxy, for the direction of motion varies with the nature and concentration of the medium. It would thus be a reaction to the "ion" liberated in contact with the one or other extremity of the being. Induction shocks, as we have seen, if slight, arrest the movements of Protozoa, or if a little stronger determine movements of contraction; if of sufficient intensity they kill them. No observation seems to have been made on the behaviour of Protista in an electric field. A magnetic field of the highest intensity appears to be indifferent to all Protista.
(vii.) We have already referred to the effect of dissolved Chemical Substances present in the water. If the substance is in itself not harmful, and the effect varies with the concentration, we term the reaction one of "tonotaxy," which combines with that of "chemiotaxy" for substances that in weak solution are attractive or repellent to the being. Paramecium, which feeds on bacteria, organisms of putrefaction, is positively chemiotactic to solutions of carbon dioxide, and as it gives this off in its own respiration, it is attracted to its fellows. The special case of reaction to gases in solution is termed "aerotaxy," or "pneumotaxy," according as the gas is oxygen or carbon dioxide. We find that in this respect there are degrees, so that a mixed culture of Flagellates in an organic infusion sorts itself out, under the cover of a microscopic preparation, into zones of distinct species, at different distances from the freely aerated edge, according to the demands of each species for oxygen and CO2 respectively.
Finally, we must note that the apparently "spontaneous movements" of Protists can hardly be explained as other than due either to external stimuli, such as we have just studied, or to internal stimuli, the outcome of internal changes, such as fatigue, hunger, and the like. Of the latter kind are the movements that result in REPRODUCTION.
Reproduction.—We have noted above that the growth of an organism which retains its shape alters the ratio of the surface area to the whole volume, so necessary for the changes involved in life. For the volume of an organism varies as the cube of any given diameter, whereas the surface varies with the square only. Without going into the arithmetical details, we may say that the ratio of surface to volume is lessened to roughly four-fifths of the original ratio when the cell doubles its bulk. As Herbert Spencer and others have pointed out, this must reduce the activities of the cell, and the due ratio is restored by the division of the cell into two.[[33]] This accounts for what we must look on as the most primitive mode of reproduction, as it is the simplest, and which we term "fission" at Spencer's "limit of growth." Other modes of reproduction will be studied later (p. [30]), after a more detailed inquiry into the structure of the nucleus and of its behaviour in cell-division. All cell-division is accompanied by increased waste, and is consequently catabolic in character, though the anabolic growth of living protoplasm, at the expense of the internal reserves, may be concurrent therewith.
Cell-Division
In ordinary cases of fission of an isolated cell the cell elongates, and as it does so, like other viscid bodies, contracts in the middle, which becomes drawn out into a thread, and finally gives way. In some cases (e.g. that of the Amoeba, Fig. 4) the nucleus previously undergoes a similar division by simple constriction, which is called direct or "amitotic" division. But usually the division of the nucleus prior to cell-division is a more complex process, and involves the co-operation of the cytoplasm; and we must now study in detail the nucleus and its structure in "rest" and in fission.[[34]]