Lessons in elementary biology / by T. Jeffery Parker.

  • Thomas Jeffery Parker
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary biology / by T. Jeffery Parker. Source: Wellcome Collection.

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    show an increased activity, then become more and more sluggish, and at about 30°—35° C. cease altogetlier, re- commencing, however, wiren the temperature is lowered. If the heating is continued up to about 40° C. the animal- cule is killed by the coagulation of its ]rrotoplasm (see p. 5); it is then said to suffer Jieat-ri^'or or death-stiffening pro- duced by heat. Similarly when it is cooled below the ordinary temperature the movements become slower and slower, and at the freezing point (0° C.) cease entirely, but freezing, unlike over-heating, does not kill the pro- toplasm, but only renders it temporarily inert ; on thawing, the movements recommence. We may therefore distinguish an optimum temperature at which the vital actions are carried on with the greatest activity; maximum and minimum tem- peratures above and below which respectively they cease ; and an ultra-maximum temperature at which death ensues. There is no definite ultra-minimum temperature known in the case of Amoeba. The quantity of water present in the protoplasm—as water of organization (see p. 5)—is another matter of importance. The water in which Amoeba lives always contains a certain percentage of salts in solution, and the protoplasm is affected by any alteration in the density of the surrounding medium; for instance, by replacing it by distilled water and so reducing the density, or by adding salt and so increasing it. The addition of common salt (.sodium chloride) to the amount of two per cent, causes Amoeba to withdraw its ]jseudopods and undergo a certain amount of shrinkage : it is then said to pass into a condition of Under these circumstances it may be restored to its normal con- dition by adding a sufficient proportion of water to bring back the fluid to its original density. In this connection it is interesting to notice that the dele-
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