A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . denser than the water in the soil, so there isa continuous flow from the latter to the former. But if,for any reason, the density of the liquids should be reversed,the flow would set in the opposite direction, and if continuedlong enough, the strength of the plant would be literally sapped by the exhaustion of its tissues, so that it woulddie. What is this process of cell exhaustion called ? 73. The use of acid secretions to the root. — It wasshown in Exp. 50 that carbon dioxide a
A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . denser than the water in the soil, so there isa continuous flow from the latter to the former. But if,for any reason, the density of the liquids should be reversed,the flow would set in the opposite direction, and if continuedlong enough, the strength of the plant would be literally sapped by the exhaustion of its tissues, so that it woulddie. What is this process of cell exhaustion called ? 73. The use of acid secretions to the root. — It wasshown in Exp. 50 that carbon dioxide and probably other sub-stances occur in the im-mediate vicinity of dioxide is an ac-tive agent in dissolvingthe various mineral mat-ters contained in the soil,and as these last can beabsorbed only in a liquidor a gaseous state (63),the advantage to theroot as an absorbent or-gan, of being able to se-crete such active sol-vents, is obvious. 74. Relation of rootsx^ OK A . 1 . u- to the soil. — In order to Fig. 85. — A natural root etching, found on a piece of slate. perform their work of ab-. THE ROOT 71 sorption, roots must have access to a suitable soil. To pro-duce the best results a soil must contain (1) all the essentialmineral constituents (62); (2) moisture for dissolving thesematerials; and (3) air enough to supply the oxygen which isnecessary to the life processes of all green plants. 75. Composition of soils. — Sand, clay, and humus, orvegetable mold, with the various substances dissolved inthem, constitute the basis of cultivated soils. A mixtureof sand, clay, and humus is called loam. When the propor-tion of humus is very large and well decomposed, the mixtureis called muck. Pure sand contains but little nourishingmatter and is too porous to retain water well. Pure clayis too compact to be easily permeable to either air or soils are composed of a mixture of the two with vege-table mold in varying proportions, giving a sandy loam, ora cla
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