If we pull or dig up a plant and, after washing its roots free from adhering soil, weigh it, then dry it thoroughly in a warm place, and weigh it again, we find that water has formed a very large proportion (often well over 90 per cent.) of the substance of the fresh plant. If we chop up the dried plant and heat the pieces strongly in a crucible or hard glass tube, theyIf we pull or dig up a plant and, after washing its roots free from adhering soil, weigh it, then dry it thoroughly in a warm place, and weigh it again, we find that water has formed a very large proportion (often well over 90 per cent.) of the substance of the fresh plant. If we chop up the dried plant and heat the pieces strongly in a crucible or hard glass tube, theyto prevent escape of water, but which in the root bear countless fine hairs which absorb water and salts from the soil, while in the leaf these tissues are perforated by small openings (stomata) through which important exchanges of gases take place between the outer air and the air-spaces within the plant. Food Synthesis.By a remarkable series of processes, green plants, alone of all living things, build up organic compounds fiom the carbon dioxide of the air and a few elements absorbed from the soil. The first step (Carbon Assimilation) is the splitting up of carbon dioxide and water, and the rearranging of the elements carbon, hydrogen, and oxygen so as to produce simple organic compounds. There are intermediate products, but finally sugars are formed. Some of the sugar is at once used up to supply energy to the green tissue itself, some passes along the conducting tissue to parts of the plant vhich are growing or are storing food, while some is temporarily changed into the allied, but insoluble substance starch, for storage in the green leaf tissue itself. Starting from sugar, various other organic compounds are built up containing (besides carbon, hydrogen, and oxygen) the elements nitrogen, phosphorus, and sulphur, and finally the living substance protoplasm itself is produced. Of these two sets of synthetic processescarbonic acid to sugar, sugar to protoplasm the first (Carbon Assimilation) depends absolutely upon light, and occurs only in the green leaf, while the second is independent of Ught, and may be carried on in any part of the plant. We may call the first photosynthesis, and the second proteidsynthesis. A little reflection shows the unique and fundamental importance of these synthetic processes, which green plants alone can carry on, and upon which all other Uving things are directly or indirectly dependent for their food, and therefore for their hfe. We know very little about the later stages, but as regards photosynthesis, we know that the green matter (chlorophyll) of leaves absorbs the red and blue rays of the mixed (white) light falling on it, and that the absorbed rays somehow supply the living protoplasm with the energy required to carry out the synthesis of sugar from carbonic acid (i.e. carbon dioxide from the air, dissolved in water derived from the soil). The vast importance of having the plants leaves well exposed to the light and air is thus obvious. Essential Elements.The reason why six out of the ten”essential elements”are essential is clear enough from the fact that they are constituents of protoplasm itself. But why are the other four also essential i As to iron and magnesium the reason is clearwithout these elements the green matter (chlorophyll) cannot be formed ; magnesium actually enters into the composition of chlorophyll, but as iron does not, the precise part played by iron is not clear. Potassium is an ingredient in certain organic compounds, found especially in some crucifers (e.g.”oil of mustard”), but apparently its chief functions are somehow connected with carbon assimilation and with the transport of organic foodstufis in the plant. Calcium compounds (lime salts) are also concerned in some way in photosynthesis and proteid-syuthesis, besides combining with, and converting into a harmless insoluble salt, a poisonous by-product (oxalic acid) formed in these synthetic processes, while its compounds also enter into the composition of, and give additional strength to, the supporting (cell-wall) framework of plant tissue. Besides the ten elements which are essential and sufficient for healthy growth of all green plants (so far as known from water culture experiments), a few others occur in considerable amount in certain plants. In some cases these additional elements certainly make for more robust growth; for instance, grasses and cereals store up silica (from quartz, flint, or sand) to stiffen their stems and leaves, while common salt (sodium chloride) seems to promote the growth of plants of the goosefoot family (spinach, beetroot, etc.), as well as some of the few edible umbelliferae, notably the carrot. Respiration.Plants, Uke animals, use organic foodstuffs as material for tissue formation in their growth, and as”fuel,”which is slowly burned or oxidised, in order to supply energy for the various functions of life. Actual locomotion, characteristic of animals (often only in early life, as in oysters and other sedentary animals), is, in the vegetable kingdom, seen only in lowly microscopic plants, and in certain reproductive cells of higher ones ; but more or less active streaming movements of protoplasm can be seen with the miscroscope in allplantsfor instance, in the green cells of leaves, the protoplasm moves so as to place the disc-like chlorophyll grains broadwise in moderate light, but edgewise in too intense light which injures the chlorophyll. Also, energy is expended in the chemical transformation undergone by the organic foodstuffs in plants, just as in animals ; these foods are oxidised, with the setting free of heat and of carbon dioxide, besides other waste products which are stored inside the plant, instead of being excreted as in animals and man. In plants, as in animals, part of the energy produced by oxidation (respiration) is set free as heat, but in plants this occurs slowly, and the heat is soon dissipated so that the tissues take the temperature of the air around ; though in germinating seeds and opening buds and flowers, respiration may be as active as in warm-blooded animals, and the temperature rise by several degrees. The oft – repeated steam – engine analogy applies to plants just as to animals or our own bodies ; and a free supply of air containing the necessary oxygen for respiration is as important for plant life as for that of animals. For instance, the functions of the root, and therefore those of the rest of the plant, suffer (i) if the soil is waterlogged (i.e. if the air between the soil grains is replaced by water, as may happen naturally in heavy clayey soil during prolonged wet weather, or in any soil if it is over-watered) ; and (2) if seeds are planted too deeply in the soil, since the lower parts of most soils contain much more water and far less air than the upper part. A well-ventilated soil then is a most important condition for the healthy growth of plants. The ground tissue of roots contains air-spaces continuous with those in stem and leaves, and this internal ventilation system is specially well developed in marsh and aquatic plants, but ordinary land plants must also have a well-ventilated soil to grow healthily in.
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