=237. The yeast plant.=—Before the caustic potash is placed in the tube some of the fermented liquid should be taken for study of the yeast plant, unless separate cultures are made for this purpose. Place a drop of the fermented liquid on a glass slip, place on this a cover glass, and examine with the microscope. Note the minute oval cells with granular protoplasm. These are the yeast plant. Note in some a small “bud” at one side of the end. These buds increase in size and separate from the parent plant. The yeast plant is one-celled, and multiplies by “budding” or “sprouting.” It is a fungus, and some species of yeast like the present one do not form any mycelium. Under certain conditions, which are not very favorable for growth (example, when the yeast is grown in a weak nutrient substance on a thin layer of a plaster Paris slab), several spores are formed in many of the yeast cells. After a period of rest these spores will sprout and produce the yeast plant again. Because of this peculiar spore formation some place the yeast among the sac fungi. (See classification of the fungi.) =238. Organized ferments and unorganized ferments.=—An organism like the yeast plant which produces a fermentation of a liquid with evolution of gas and alcohol is sometimes called a _ferment_, or _ferment organism_, or an _organized_ ferment. On the other hand the diastatic ferments or enzymes like diastase, taka diastase, animal diastase (ptyalin in the saliva), cytase, etc., are _unorganized_ ferments. In the case of these it is better to say _enzyme_ and leave the word ferment for the ferment organisms. =239. Importance of green plants in maintaining purity of air.=—By respiration, especially of animals, the air tends to become “foul” by the increase of CO₂. Green plants, i.e., plants with chlorophyll, purify the air during photosynthesis by absorbing CO₂ and giving off oxygen. Animals absorb in respiration large quantities of oxygen and exhale large quantities of CO₂. Plants absorb a comparatively small amount of oxygen in respiration and give off a comparatively small amount of CO₂. But they absorb during photosynthesis large quantities of CO₂ and give off large quantities of oxygen. In this way a balance is maintained between the two processes, so that the percentage of CO₂ in the air remains approximately the same, viz., about four-tenths of one per cent, while there are approximately 21 parts oxygen and 79 parts nitrogen. =239a. Comparison of respiration and photosynthesis.=