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.
| Starch formation or Photosynthesis. | Carbon dioxide is taken in by the plant and oxygen is liberated. |
Starch is formed as a result of the metabolism, or chemical change. | |
The process takes place only in green plants, and in the green parts of plants, that is, in the presence of the chlorophyll. (Exception in purple bacterium.) | |
The process only takes place under the influence of sunlight. | |
It is a building-up process, because new plant substance is formed. | |
| Respiration. | Oxygen is taken in by the plant and carbon dioxide is liberated. |
Carbon dioxide is formed as a result of the metabolism, or chemical change. | |
The process takes place in all plants whether they possess chlorophyll or not. (Exceptions in anaerobic bacteria). | |
The process takes place in the dark as well as in the sunlight. | |
It is a breaking-down process, because disintegration of plant substance occurs. | |
[CHAPTER XII.]
GROWTH.
By growth is usually meant an increase in the bulk of the plant accompanied generally by an increase in plant substance. Among the lower plants growth is easily studied in some of the fungi.
240. Growth in mucor.—Some of the gonidia (often called spores) may be sown in nutrient gelatine or agar, or even in prune juice. If the culture has been placed in a warm room, in the course of 24 hours, or even less, the preparation will be ready for study.
241. Form of the gonidia.—It will be instructive if we first examine some of the gonidia which have not been sown in the culture medium. We should note their rounded or globose form, as well as their markings if they belong to one of the species with spiny walls. Particularly should we note the size, and if possible measure them with the micrometer, though this would not be absolutely necessary for a comparison, if the comparison can be made immediately. Now examine some of the gonidia which were sown in the nutrient medium. If they have not already germinated we note at once that they are much larger than those which have not been immersed in a moist medium.
242. The gonidia absorb water and increase in size before germinating.—From our study of the absorption of water or watery solutions of nutriment by living cells, we can easily understand the cause of this enlargement of the gonidium of the mucor when surrounded by the moist nutrient medium. The cell-sap in the spore takes up more water than it loses by diffusion, thus drawing water forcibly through the protoplasmic membrane. Since it does not filter out readily, the increase in quantity of the water in the cell produces a pressure from within which stretches the membrane, and the elastic cell wall yields. Thus the gonidium becomes larger.