A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . be weighed anda record kept for com-parison with the seed-lings at the end of theexperiment. Plant thetwo sets in pots con-taining exactly thesame kind of soil, andkeep under identicalconditions as to light,temperature, andmoisture. Keep theseedlings under obser-vation for two or threeweeks, making dailynotes and occasionaldrawings of the heightand size of the stems,and the number ofleaves produced byeach. 33. Resistanceto heat and cold.—In making experi-ments with regard to tempe
A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . be weighed anda record kept for com-parison with the seed-lings at the end of theexperiment. Plant thetwo sets in pots con-taining exactly thesame kind of soil, andkeep under identicalconditions as to light,temperature, andmoisture. Keep theseedlings under obser-vation for two or threeweeks, making dailynotes and occasionaldrawings of the heightand size of the stems,and the number ofleaves produced byeach. 33. Resistanceto heat and cold.—In making experi-ments with regard to temperature, notice how the extremestolerated are influenced, first, by the length of time theseeds are exposed; second, by the amount of water containedin them; and third, by the nature of the seed coats. Everyfanner knows that the effect of freezing is much more in- 50 Figs. 50, 51. — Stem development of seedlings: 50,raised from healthy grains of barley; weight, (about 500 grs.) ; 51, raised under exactly similarconditions from the same number of inferior grains;weight, 23 grams (about 350 grs.).. 52 53 Figs. 52, 53. — Improvement of corn by selection :62, original type; 53, improved type developed from it. 38 PRACTICAL COURSE IN BOTANY jurious to plants or parts of plants when full of sap (water)than when dry. This, in the opinion of the most recentinvestigators, is because the water in the spaces outside thecells freezes first and as moisture is gradually withdrawnfrom the inside to take its place, the soluble salts which maybe present in the cell sap become more concentrated, and bytheir chemical action on the contained proteins cause themto be precipitated, or salted out, as we see sugar or saltprecipitated from solutions of those substances when wateris withdrawn by evaporation. In this way, it is believed,the fundamental protoplasm of the cell may be so disorganizedthat death ensues if the freezing is continued long enough,since the protein precipitates beco
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