PRACTICAL IRRIGATION

Otago Witness, Issue 3018, 17 January 1912, Page 22

PRACTICAL IRRIGATION

By John M'Keague.

PLANT LIFE.

I. (Copyright) In order to understand the effects of irrigation in the soil and on the life and growth of plants it is necessary first to understand how plants germinate, live, grow, and reproduce themselves; and, further, what fresh conditions and effects are produced in the soil by the presence of irrigating water. Some plants, as the mistletoe, live and grow in air only; others in water only. But the soil is the birthplace and the home of nearly all plants, and from the soil and the air all plant life derives its daily food. Some plants are reproduced from cuttings, but most new plant life is derived from seeds. Plants and seeds are separated into various classes, but for our purpose, which is to understand the origin, the machinery, and the conditions necessary for the growth, the development, and the reproduction of plants, it is necessary only to understand the facta in relation to two common seeds —the pea and the seed of wheat. Take a pea and carefully examine it. Around the outside is a skin of dead matter. Remove the akin, and the remaining part will be found to consist of two thick masses placed face to face; and, further, these two thick masses are united by a. long-shaped body, which is joined to each mass. This longshaped body is the embryo, and the thick masses furnish food for the germinated and growing young plant. Examine now the seed of wheat. The embryo here consists of one thick mass, which is placed at one side; the other sido consists of a white floury substance. Take either of these seeds" and place it in dry mould. So long as the mould remains dry the seed will not germinate Moisten the mould, but keep the seed and the mould at or below the freezing-point, and the seed will not germinate. Now place the seed in moist mould and in a temperature above the freezingpoint, but exclude the air, and still the seed will not germinate. But if the air he allowed to come in contact, with the seed and to circulate through the moist, warm mould, the seed will germinate. Large quantities of oxygen, which is one of the gases that form the air, are consumed by germinating seeds, and seed will not germinata in th© absence of oxygen, and hence the air must find its way freely to the germinating seed. The combination of oxygen with other elements is termed oxidation It thus appears that three conditions are necessary for the germination of f^eed —(a) moisture, (b) warmth, (c) air. When the seed commences to grow one end of the long-shaped body grows upwards and forms the etem, and afterwards the branches, leaves, buds, and flowers of the new plants The other end grows downwards and forms the root. In many cases the root descends to a groat depth in the soil, and is known as the taproot, as in tho case of the pea and other plants of the same family. Rootlets usually grow and spread in all directions from the taproot. In the case of wheat and other sur-face-growing plants there is not a taproot. Rootlets instead grow and spread through, tho soil which surrounds the seed. The ssft points of growing roots do not bore through solid clods or firmly-packed earth. They push through the open spaces between the particles of the soil, grow rapidly, and the hairs' on the rootlets cling to tho particles of the soil in which the plant, as a whole, is firmly fixed. These hairs perform an important function in- the nourishment of the plant. When tlie embryo has germinated and commences to grow the young plant at first T-cceives its nourishment from the thick masses to which the embryo was, and still is, attached. When the root is firmly fixed in the soil tho plant food- is no longer supplied by the seed, but is taken directly from the soil through the roots and from tho air through the leaves. As soon as this occurs two fresh conditions, in addition to those necessary for the germination of the seed, are needed for the growth and development of the plant—-namely, certain portions of the soil and light. The principal parts of most plants are -(a) the root, (b) the stem or trunk, (e,) the leaves, (d) the flower, and (e) the fruit, which usually contains the seed. Plach of these parts is called an organ. Each organ has sceoial work to do, and tliat vork is called Die function of the organ. The function of the root is -to hold the plants firmly in the soil, to take up nourishment for the plant from the eoil, and sometimes to store up foodstuffs for tho use of the plant, as i/i the caees of the pea, wlieat, turnip, mangold, and the fruits. The function of the stem or trunk is to support the branches, leaves, buds, and flowers, and to act as a channel through which to convey the nourishment from the roots to the branches, the leaves, the flowers, and the fruit. The function of the leaves is to piovide a large surface for exposing th', l plant "ood to the action of the sunlight and heat, and to absorb caibonic acid gas from tho air. The functions of the flower and the sied are to bring about the reproduction of the plant. The substance of which plants, and the vegetable world in general-tare formed is called tissue. This tissue consists of minute cells. Each cell is a very small round body, and consists of a transparent outside skin, which encloses, in the living plant, the plant food. When the cell is dead the inside or interior contains air only. The plant-food, in the living cell, consists of an active. living substance culled protoplasm. As the pi;ml grows the cells enlarge, the protoplasm gathers into a small round body in the centre of the cell. Fibres spread out from this small round bodv, and attach themselves to the side of the cell. In time the cell divides, and each portion forms a new cell. By this process* of enlargement and division of the cells the ;>lant grows am] increases until it arrives at maturity. The leaves aro continuous with the skin or bark of rhe plant or tree, and their formation is .simple. Lav a green leaf on a flat surface, and cut it in two with a sharp knife. Now. examine the cut edge through a microscope. It will be seen that a thin and delicate s-kin covers the leaf on each side, and immediately below the skin is a layer of closely-packed cells. Towards tho interior of the leaf Kvexal Uivers of loosely packed cells are seen. The t-kin of the leaf has many minute openings, which open more widely in sunlight

than in darkness. The tissue of the plant proper is thus in complete union with the tissue of the leaf, and the circulation is comcictc and continuous from the roots to the "leaves. The stem and the branches of a plant or tree are usually divided at fairly regular distances into sections. Where these join, a raised ring runs around the stem of the branch These raised rings are called nodes, and it i- from these rings that the leaves arise During autumn buds are formed, either at the ends of the stems and branches or at these nodes. J he bud itself, and its circulation, are a so continuous with the tissue of the stem. They are often protected from coid by a covering of scales, of resin, of gum, or of hairs. During waiter they remain as they were formed in aut-urw. Growth begins in the spring, and the buds develop into leafy branches, which may form either leaves or flowers, or both. Instead of only growing longer, they may grow in width. When the bud grows into a flower, the object is to bring about the reproduction of the, plant by reproducing seed. Flowers are of many shapes ami forms. A perfect flower consists of (a) the calyx, (b) the corolla, (c) the stamere, (d) the" pistil. From the p°i nt where the flower joins the stalk, separate pieces, generallv of a green colour, form a row around the outside of the .flower This outside row is the calyx. Within the calyx is a second ring of separate pieces. I hose are nearly always white, or coloured, hardlv ever green. This row !s called the corolla. It secretes a eugary juice, called nectar, which attracts insects to the flower Within the corolla or second row is a third row usuallv formed of slender stalks, called stamens, and on the top of each stalk is a small vessel, which holds the powder known as pollen that is necessary for fertilising the seed The fourth ring assumes more forms than any other part of the flower. Its I simplest form is the flower of the pea. I The pistil of the common pea is simply a leaf folded down the middle, with its edges united to form a hollow vessel called the ovary, or vessel which contains the seed. If a flower has all these rows, it is a perfect flower. If any row be absent, the flower is imperfect. The use of the ovary is two-fold. It secretes little grains within its cavitv. It also provides the means for conducting the pollen from the stamens to its interior.- The pollen may be conducted to the ovary either by direct contact of stamen with pistil by insects, or by the action of the wind. When the pollen has reached the cavity of the ovary the little grains in that cavity are fertilised, and become seed, fitted, in due course, to reproduce the plant. The food of the plant is derived from two sources—from the soil, through its roots; and from the air, through its leaves. The hairs of the rootlets, as we have seen, cling to particles of the soil. The soil is a laboratory in which solutions of plant food are compounded. The plant, and especially the roots, exude acid salts, which have the power, aided by water and carbonic acid, to dissolve and bring into the plant various useful matters that were previously lying outside the roots insoluble in water. The hairs of the rootlets are formed of cells; or exceptionally minute bags The outer skin of the cell or bag is called the walls of the cell. These parts . of the liquid solution thus formed in the soil, which are suitable for the food of the plant pass through the walls of the ceh, and bv the same proce.s—much the same as that by which oil ascends a wick. The leaves absorb air through the small openings or pores. Inside the leaf, the air is resolved into nitrogen and oxygen. The oxygen is liberated and the nitrogen incorporated with the plant food, which has arrived from the roots. The whole is exposed to the action of sunlight, and converted into plant food, much after the manner a housewife makes bread from flour, milk, salt, and yeast. The housewife is Nature acting in the shape of sunlight; the flour, the salt, and yeaet are provided bv the conjoint action of the plant as the carrier from mill or laboratory in the soil; and the manufactured plant food is sent to every living part of the 1 plant to be uwd for the growth, the sustenance, and the reproduction of the plant, in much the same way as a hungry schoolboy eats, up his dinner. As the schoolboy wants water or tea with his food so the ripnt requires water. The water ascends through the roots, stem, branches. and arrives at the leaves, not by passing from cell to cell, as the plant food does, but chiefly through the fibrous parts of the plant; and it goes through these parts more rapidly than it could go from cell to cell; and the transmission of both plant food and water is much more rapid in sunlight than in darkness. There are thus two movements constantly going on in the living, healthy plant—(a*) the movement of the plant food from cell to cell, and (b) the movement of water from the roots to the leaves. The normal movements of the plant food are both upwards and downwards from the factory or laboratory in the leaves. All growing plants and trees require a very large amount of water. Turnips contain over 90 por cent., or 93 parts out of every 100 parts, of water, and Avatercress 96 per cent. The water, having passed through the plant or tree, is exhaled by the leaves. If a window sash or other framed glass be laid on grass the glass speedily becomes clouded from the Avater escaping from the grass. Even in the driest seasons, if a cold bell-glass be placed over growing grass, water enough to trickle down the sides of the glass will be deposited in a few minutes. An acre of grass exhales a large number of hogsheads of water in a day. Great masses of any foliage give off great quantities of water daily and prevent stagnation. Most plants that, farmers cultivate use up over 200 times their dry weight of water. A single plant of barley, in full growth and vigour, requires the passage through it while in tho soil of more than a gallon of water. An acre of cabbages, in full growth, will use more than 10 tons of water in 12 hours. It is very evident, then, that if water in large quantities be not available for growing plants their growth and development are etunted, and imperfect crops, resulting in serious loss, are the consequence. The evaporation of the water from tho leaves keeps the plant cool, even in the hottest weather. If a plant thus constructed a-nd nourished be placed where it cannot receive the sunlight and heat it becomes of a sickly white colour. During the process of manufacturing and oxidizing the plant food in the plant laboratory a certain amount of heat is evolved by the oxidation within the plant, but that heat will not prevent the plant, in the absence of sunlight, from losing its colour. But if placed where it can receivo sunlight, it quickly presents a healthy and vigorous appearance, and becomes, while growing, mostly of a green colour. The

.sunlight acts on the carbonic acid in the leaves, and carbon., without which no green plant can grow. The foliage of young plants cannot oven exist for any length of time when exposed to sunlight in air that is totally free from,carbonic acid. Carbonic acid enters the leaves very much as the liquids enter the roots. The amount of carbonic acid decomposed depends on the intenisty of the light; and the prosperity of the plant depends largely upon the amount of light it receives. ' "When a ray of sunlight pae-sos through a threesided piece of glass it is found to consist of h, number of differently-coloured rays. When the sunlight enters the leaves or the flowers of plants, the particles of which the leaf or the flower is formed, act on the light exactly as the three-sided piece of glass acted.' Tlie ray h resolved into the dif-ferently-coloured rays of which «t was composed, one of th-a rays being reflected to the eye of the observer, and the remaining rays absorbed by the plant. If the rod ray he reflected, the leaf or the flower will appear of a red colour; if the green ray be reflected, the colour of the 'oaf or the flower will be green. In thi s way sunlight colours the grass, the haves, and the flowers. But the sunlight does more than produce colour. The sunbeam contains heat rays, in addition to the rays of colour. These heat rays, in conjunction with the carbonic acid gas. act on the plant food, and ; chiefly in the day time, convert the plant food into starch. Largely in the nightirne this starch is converted into sugar and other nourishing substances, and sent to all parts of the living, healthy plant, to nourish it. If a plant, thus produced from a seed, bo placed on a red-hot p'ate of iron,, most of it will vanish into air. and a small part, the ashes, will remain on the plate. The portion of the plant that vanishes is called the organic part, and the ashes that remain on - the plate are called the inorganic part. If a growing plant of 1001 b in weight be burnt, it will usually be found that 951 b wil' thus vanish, and only about 51b of ashes will remain on the plate. The organic part of the plant which vanish consists chiefly of woody fibre, starch, sugar, gum, oil, and fat. Chemists can reduce ali these into gases, or into substances, much simpler than themselves. The chief of these are oxygen, hydrogen, nitrogen, and carbon. Oxygen and nitrogen are the gases which form the air we breathe. Oxygon and hydrogen form water. The inorganic pUrt of a plant consists mostly of lime, magnesia, soda, potash, iron, manganese, sand, iodfhe. and various acids. It thus appears that plants consist of the ordinary gases of nature, and the ordinary substances of the soil. Wo shall endeavour to show how all these substances hnd their way into the organism of the plant. The pressure of moiswire in the atmosphere alone is of little avail for the successful growth of crops. The vapour must be condensed, fall in rain, and, penetrate the soil. The three chief condensers are —(a) trees -and forests, (b) hills and mountains, (c) cold derived from any source. Many instances could be given, such as at Honolulu and Trinidad, where forests originally existed, and until these were- cut down the rainfall was sufficient and regular. After the forests were cut down the rainfall lji.rge.ly decreased, and the rain fell at irregular intervals. No sooner had trees been extensively grown again than the rainfall again increased in quantity and became more regular. In New Zealand mountains and forests condensa the vapour in the province of Westiand, and a rainfall of from 92 to 115 inches or more is tlie result. In the province of Auckland these condensers are largely absent, and the consequence is a low rainfall of from 3o to 53 inches yearly. In Australia a range of mountains runs, roughly, parallel with the east coastline, and usually at some distance inland. Between the mountains and the ooastli.ne_ the rainfall is high and regular, while on the vast plains of the interior it is low and irregular. Where cold air currents -exist, whether the result of permanent atmospheric conditions, or induced by the presence of large sheets of water, then the influence of such condensers in contracting, the air, and hence causing it to precipitate its vapour in the form of rain, is great and apparent.