A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . lacking. 63. How plants obtain their food material. — Plantsobtain their supply of the various mineral salts from solu-tions in the soil water whichthey absorb through theirroots. With a few doubtfulexceptions, they cannot as-similate their food unless itis in a liquid or gaseous the gases, carbon dioxide,oxygen, and hydrogen canbe freely absorbed from theair, or from water with va-rious substances in solution,but most plants are so con-stituted that they cannot absorb fre
A practical course in botany : with especial reference to its bearings on agriculture, economics, and sanitation . lacking. 63. How plants obtain their food material. — Plantsobtain their supply of the various mineral salts from solu-tions in the soil water whichthey absorb through theirroots. With a few doubtfulexceptions, they cannot as-similate their food unless itis in a liquid or gaseous the gases, carbon dioxide,oxygen, and hydrogen canbe freely absorbed from theair, or from water with va-rious substances in solution,but most plants are so con-stituted that they cannot absorb free nitrogen from the air;they can take it only in the form of compounds from nitratesdissolved in the soil, and hence the importance of ammoniaand other nitrogenous compounds in artificial of the pea family, however, bear on their roots littletubers formed by minute organisms called bacteria, whichhave the power of extracting nitrogen directly from thefree air mingled with the soil; and hence the soil in whichthese tuber-bearing legumes decay is enriched with niirogenin a form ready for i. Fig. 75. — Roots of soy bean bearingtubercle-forming bacteria. THE ROOT 61 Practical Questions 1. Could any normal plant grow in a soil from which nitrogen was lack-ing? Potash? Lime? Phosphorus? (62.) 2. Could it live in an atmosphere devoid of oxygen ? Nitrogen ? Car-bon dioxide? (62.) 3. Why are cow peas or other legumes planted on worn-out soil to renewit? (63.) 4. Is the same kind of fertilizer equally good for all kinds of soil ? Forall kinds of plants ? (60, 62.) 5. Why does too much watering interfere with the nourishment ofplants? (Exps. 26, 27.) 6. Are ashes fit for fertilizers after being leached for lye? (62.) 7. Why will plants die, or make very slow growth, in pots, unless thesoil is renewed occasionally? (60, 62.) III. STRUCTURE OF THE ROOT Material. — Taproot of a young woody plant not over one or twoyears old; apple and cherry shoots make good spec
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