d. Pasteur's fluid with sugar. [e. Mayer's pepsin solution1.] Keep all at about 35° C., and compare the growth of the yeast, as measured by the increase of the turbidity of the fluid, in each case. "a" will hardly grow at all, "b" better, "" better still, "d" well, and "e" best of all. Note that bubbles of gas are plentifully evolved from both the solutions which contain sugar. That any growth at all takes place, in the case of experiments a and b, is due to the fact that the drop of yeast added contains nutritious material sufficient to provide for that amount of growth. 2. 3. 4. Prepare two more specimens of "d" and keep one in a cold-the other in a warm (35° C.) place, but otherwise under like conditions. Compare the growth of the yeast in the two cases; it is much greater in the specimen kept warm. Prepare two more specimens of "d"; keep both warm, but one in darkness, the other exposed to the light: that in the dark will grow as well as the other; sunlight is therefore not essential to the growth of Torula. Sow some yeast-cells in Pasteur's solution in a flask, the neck of which is closed by a plug of cotton wool, and boil for five minutes; then set it aside; no signs of vitality will afterwards be manifested by the yeast in the flask; it is killed by exposure to this temperature. 1 Mayer's solution (with pepsin)= 15 per cent. solution of sugar-candy Magnesic sulphate.. Pepsin...... 20 CC. ......... o' I grm. O' I grm. o' I grm. O'23 grm. 5. [Take two test tubes; in one place some yeast, with Pasteur's solution containing sugar; in the other place baryta water, and then connect the two test tubes by tightly fitting perforated corks and a bent tube passing from above the surface of the fluid in the first tube to the bottom of the baryta water in the second; pass a narrow bent tube, open at both ends, through the cork of the baryta water tube, so that its outer end dips just below the surface of some solution of potash1. All gas formed in the first tube will now bubble through the baryta water in the second, and, from thence, any that is not absorbed will pass out through the potash into the air. An abundant precipitate of barytic carbonate will be formed which can be collected and tested. The fermenting fluid, therefore, evolves carbonic anhydride.] 6. [Grow some yeast in Pasteur's solution (with sugar), in a nearly closed vessel (say a bottle with a cork through 7. [Determine that heat is evolved by a fluid in which active alcoholic fermentation is going on. Place 200 cc. of fresh yeast in a flask, and add 1 litre of Pasteur's fluid with sugar: put another litre of the fluid alone in a similar flask, cover each flask with a cloth and place the two side by side in a place protected from draughts. When gas begins to be actively evolved from the yeastcontaining solution, take the temperature of the fluid in each flask with a good thermometer; the temperature of the one in which fermentation is going on will be found the higher.] 1 The object of the potash is to shield the baryta water from any carbonic anhydride that may be in the atmosphere. IF the mud which accumulates in roof-gutters, waterbutts, and shallow pools, be collected, it will be found to contain, among many other organisms, specimens of Protococcus. In one of the two conditions in which it occurs, Protococcus is a spheroidal body to 10000 of an inch in diameter, composed, like Torula, of a structureless tough transparent wall, inclosing viscid and granular protoplasm. The chief solid constituent of the cell-wall is cellulose. The protoplasm contains a nitrogenous substance, doubtless of a proteinaceous nature, though its exact composition has not been determined, and indications of starchy matter are sometimes to be found in it. Either diffused through it, or collected in granules, is a red or green colouring matter (Chlorophyll). Individual Protococci may be either green. or red; or half green and half red; or the red and green colours may coexist in any other proportion. In addition to the single cells, others are found divided by partitions, continuous with the cellulose wall, into two or more portions, and the cells thus produced by fission become separate, and grow to the size of that form from which they started. In this manner Protococcus multiplies with very great rapidity. Multiplication by gemmation in the mode observed in Torula is said to occur, but is certainly of rare Occurrence. The influence of sunlight is an essential condition of the growth and multiplication of Protococcus; under that influence, it decomposes carbonic anhydride, appropriates the carbon, and sets oxygen free. It is this power of obtaining the carbon which it needs from carbonic anhydride, which is the most important distinction of Protococcus, as of all plants which contain chlorophyll, from Torula and the other Fungi. As Protococcus flourishes in rain-water, and rain-water contains nothing but carbonic anhydride, which it absorbs along with other constituents of the atmosphere, ammonium salts (usually ammonium nitrate, also derived from the air) and minute portions of earthy salts which drift into it as dust, it follows that it must possess the power of constructing protein by rearrangement of the elements supplied to it by their compounds. Torula, on the other hand, is unable to construct protein matter out of such materials as these. Another difference between Torula and Protococcus is only apparent: Torula absorbs oxygen and gives out carbonic anhydride; while Protococcus, on the contrary, absorbs carbonic anhydride and gives out oxygen. But this is true only so long as the Protococcus is exposed to sunlight. In the dark, Protococcus, like all other living things, undergoes oxidation and gives off carbonic anhydride; and there is every reason to believe that the same process of oxidation and evolution of carbonic anhydride goes on in the light, but that the loss of oxygen is far more than covered by the quantity set free by the carbon-fixing apparatus, which is in some way related to the chlorophyll. The still condition of Protococcus, just described, is not the only state in which it exists. Under certain circamstances, a Protococcus becomes actively locomotive. The protoplasm withdraws itself from the cell-wall at all but two points, where it protrudes through the wall in the form of long vibratile filaments or cilia, and by the lashing of these cilia the cell is propelled with a rolling motion through the water. The movement of the cilia is so rapid, and their substance is so transparent and delicate, that they are invisible until they begin to move slowly, or are treated with reagents, such as iodine, which colour them. Not unfrequently the cell-wall eventually vanishes, and the naked protoplasm of the cell swims about, and may undergo division and multiplication in this state. Sooner or later, the locomotive form draws in its cilia, becomes globular, and, throwing out a cellulose coat, returns to the resting state. For reasons similar to those which prove the vegetable nature of Torula, Protococcus is a plant, although, in its locomotive condition, it is curiously similar to the Monads among the lowest forms of animal life. But it is now known that many of the lower plants, especially in the group of Alga, to which Protococcus belongs, give rise, under certain circumstances, to locomotive bodies propelled by cilia, like the locomotive Protococcus, so that there is nothing anomalous in the case of Protococcus. Like the yeast-plant, Protococcus retains its vitality after it has been dried. It has been preserved for as long as two years in the dry condition, and at the end of that time has resumed its full activity when placed in water. The wide distribution of Protococcus on the tops of houses and elsewhere, is thus readily accounted for by the transport of the dry Protococci by winds. |