What is Hydrotropism, Hydrotropism in Planst, How Does Hydrotropism Work
Roots respond similarly to the presence of water, turning toward moisture even at long distances. This tendency, called hydrotropism, is very useful, especially if soil water be scant. Vast numbers of fine roots are often found projecting into springs and streams, forcing their way into water pipes or piercing deep into the soil, led by this force that turns them toward the needed moisture.
What is Geotropism?, How does geotropism work, Geotropism in Plants
In order that roots may always grow where they can best absorb food materials, they show a tendency always to grow downward, i.e., toward the earth. This might at first thought he credited to mere weight, but it is evident that stems, though equally heavy, cannot be made to grow down, and that roots, though lighter than the soil, still force their way through it, and cannot be made to grow upward, even though repeatedly started in that direction.
This turning of roots and stems is caused by the attraction of the earth, called gravitation, and this response that plants make to gravitation is called geotropism —positive in the case of roots, and negative in the case of stems. Positive geotropism plays an essential part in absorption by causing the roots to penetrate the soil rather than grow in any chance direction.
Root Hairs. It has been estimated that there may be a total length of a mile in the roots of a corn plant, and alfalfa roots have been found to extend twenty feet deep in dry soil.
For the purpose of absorbing as much as possible, the surface of the active parts of all roots is covered with root hairs. These are outgrowths of the epidermal cell walls and increase the total absorbing surface enormously. They also enable the osmotic membrane to almost touch the film of water, which, even in the driest soils, clings close to the soil grains.
So important are these root hairs that their injury or loss might mean death to the plant, hence they are never borne at the extreme tip of the root, where its growth through the soil would strip them off, but are found a little back from the tip and extending various distances along the younger roots.
As the root grows, new hairs are produced near the tip, to gather moisture from new areas; the upper ones die away; the cortex and epidermis thicken, cease active absorption, and become protective in use. In frequent cases, the root hairs secrete a weak acid which helps in dissolving soil substances and in penetrating hard earth.
The adaptations of root hairs may be summarized as follows:
Osmosis. The water to supply these absolutely essential needs comes from the soil, often apparently dry, but always containing at least a little moisture which the plant must obtain if it is to live.
This vastly important root function of absorption depends on a physical process called osmosis which may be defined as the mixing or diffusion of two liquids or gases of different densities, through a non-porous membrane — the greater flow being toward the denser substance.
Osmosis is one of the most important biologic processes, and upon it depends not only absorption in roots, but all forms of absorption in plant and animal, all digestive processes, excretion, respiration, and assimilation. Wherever a liquid or gas passes through any tissue, osmosis is the acting cause, controlled sometimes by the living protoplasm that lines the cell.
The essentials for osmosis are a dense liquid, a less dense liquid, and the osmotic membrane. In the root the protoplasmic layer lining the walls of the root hairs, acts as the membrane, the cell sap as the denser, and the soil water as the less dense liquid.
Turgescence. When a plant is deprived of water, its leaves droop and we say it wilts. This is due to the fact that, normally, each cell is expanded by the water within it and so is kept in position.
If the water be withdrawn, these cells will collapse like an empty balloon, allowing the leaves and plant to droop. If water be supplied before the protoplasm dies, however, the leaves and plant will resume position.
This stiffness of plants, due to presence of water, is called turgescence and is very important in supporting the smaller plants whose stems are not stiffened with wood fibers. Nearly all leaves depend on this water pressure for their expansion.
All living matter depends more or less on liquids of various sorts, and the plant, like the animal, has its circulating fluids, bearing nourishment and removing waste, storing food, and supplying oxygen to convert that food into living energy.
From the delicious juices that flavor the peach and sweeten in the heart of the sugar cane, to the bitter milk that flows in the dandelion or lures the unwary to death in the poisonous mushroom, all consist largely of water, absorbed from the soil by the action of the roots.This absorbed water is of threefold value to the plant.
It supplies a very necessary portion of the plant's food, as water itself and as mineral matter dissolved in that water; it acts as a means of transfer within the plant for the various foods needed in the different parts, much like the blood of animals; and this absorbed water supports many parts of the plant. This latter statement will need some explanation.
The usual place from which roots develop is the lower end of the hypocotyl. Such roots are called normal roots. If they grow from other places such as the stem, leaves, or upper
part of the hypocotyl, they are called adventitious roots.
Soil Roots
Of all forms of normal roots, the commonest are the soil roots and these are of many kinds, depending upon what functions they must perform and the character of the soil, moisture,
or climate that surrounds them. They in turn may be divided into three general classes.
Fibrous Roots are made up of many fine slender rootlets, giving large extent of surface for absorption. The roots of the grasses, for instance, are so numerous that they hold the soil together, forming a compact layer called the " turf."
Tap Roots are greatly enlarged primary roots which enable the plant to go deep after water supply and hold firmly in the ground. The thistle, dandelion, burdock, and many more of our worst weeds are thus adapted to make a living under adverse circumstances.
Fleshy Roots are adapted for storage of food stuffs and have the main part greatly thickened, as in the carrot, turnip, and beet. They are generally found in plants which require two seasons to mature their seed and so need stored food to carry them over the winter. In other cases, as the dahlia and sweet potato, the fleshy root is used to reproduce the plant.
Aerial Roots. Some tropical orchids which live attached to trees and never reach the earth at all develop aerial roots. They have a very thick, spongy cortex, which absorbs water from the moist air of the forests.
Aquatic Roots. These are found in a few floating plants such as the duck-wcted and water hyacinth. They are usually small, few in number, and lacking in root hairs. They do not need
extra surface for absorption because they are surrounded by an abundant water supply.
Brace Roots
From the stems of corn and many other grasses, develop brace roots, which help to support the slender stems or to raise them again if they are bent down.
Roots for Propagation
In certain plants if the stem lies in contact with the soil for a sufficient length of time, roots will spring from the joints and produce new plants. The stems of various berry bushes can thus be fastened to the earth — " staked down " —and will take root in this way. The new root systems, when sufficiently developed, can be separated from the parent plant to make a new berry bush.
Slips or cuttings from certain plants develop adventitious roots from the stem or leaves and start new plants by this means. Many plants, like the strawberry, send out horizontal stems called " runners " from which adventitious roots develop and produce other individuals.
Climbing Roots
The stems of poison ivy, trumpet creeper, and some other vines grow climbing roots which act chiefly as means of support. These plants have ordinary soil roots, also, for the purpose of absorption.
Parasitic Roots
In a few plants, such as the dodder and mistletoe, parasitic roots develop from the stem, penetrate into the tissue of some other plant, and absorb food from their victim, often causing its death or serious injury. The dodder is parasitic upon clover, golden-rod, and other plants; the mistletoe usually grows upon the oak.
From the foregoing it is evident that roots must be profoundly varied in structure and form to perform the different functions mentioned. And it must be remembered that not only function, but other factors such as climate, soil, moisture, and exposure, which together make up the plant's environment, affect growth. We shall learn that only so far as a plant is fitted to its environment will it thrive.
Absorption. The root, as is evident from its structure, is primarily an absorbing organ, and this function will be taken up at length. However, it has many other uses and is adapted to perform very different duties in different plants.
Fixation. A second use, common to nearly all roots, is that of attaching the plant to the soil, and holding it in an upright position.
Storage. Frequently the root has sufficient bulk to act as a very efficient storage place for foods. This is particularly important for plants that retain life through long winter months.
Propagation. It may happen that enough nourishment is stored so that the plant can send up shoots at various places or even be divided, so reproducing the plant.
To prove that these food stuffs must be digested before they can be used in germinating plants, corn seeds can be tested for starch and for grape sugar, both before and after germination has started.
Starch is insoluble in cold water, and does not pass readily through the absorbing membranes. Therefore it has to be digested (changed to soluble sugars) before the plant can use it.
This digestive change is accomplished by a substance in the seed, called diastase, which acts somewhat like the digestive fluids in our bodies.
If the corn be tested before germination has begun, much starch and little or no sugar will be found. If it be tested in the same ways, after germination has proceeded for a few days, the reverse will be discovered, as most of the stored starch will have been converted into soluble form, sugar, by the diastase in the cotyledon.
The necessity of this stored food can be shown by taking a number of well-started seedlings, removing part of the stored food (in cotyledon or endosperm) in some of them, removing it all in others, and leaving still others unharmed. If these seedlings are then placed so that the root can dip into water, by suspending them on a netting over a well-filled glass, their development can be watched.
Several seedlings must be used in each group, lest we draw conclusions from too few instances, or perhaps be misled in case some one seed were abnormal. The conditions of growth must be the same in each case, lest it appear that these varying conditions, and not the loss of stored food, produces the results.
After a few days it will be seen that the whole seeds grow well and rapidly; that those with part of their food removed start more slowly and soon cease growing; while those with all the stored food removed scarcely start at all. This is because of the fact that, until the seedling can develop roots and leaves, it depends solely on this store of food whose removal is shown to have so serious results.
Germination consists of three steps, -emergence from the seed coats, penetration of the soil, and the obtaining of first nourishment.
In getting out of the seed coats, the hypocotyl appears first, emerging by way of the micropyle. The rest of the embryo follows by various ingenious schemes, all apparently planned by Nature to enable the seedling to escape uninjured from the testa, on whose protection it has so long depended.
Penetration of the soil may be either from above or from below. When seeds are scattered on the surface of the soil they are enabled to gain a foothold in the earth by various contrivances so that the Toots may be sent down into the soil. In the case of buried (planted) seed the process of penetration not only has to do with sending down roots, but the seed must find a way out of the earth, unharmed by its passage. This latter problem is solved most often by the plantlet being started from the seed in an arched position. One end of the arched stem takes hold of the ground and sends out roots, while the other, attached to the wide cotyledons or the delicate plumule leaves, gently pulls these through the ground after the growing arch has broken away to the surface. If forced directly upward these bulky appendages would be stripped off »by soil pressure.
This arch may be caused by the weight of the cotyledons and soil (as in the case of the bean), which hold back the bulky end of the plantlet until the stem is strong enough to lift it out of the ground, or (as in the case of the pea) by the tip of the plumule being held tightly between cotyledons that are not lifted frop the ground at all. In the latter case the hold of the cotyledons weakens after its store of food has been partly exhausted and the plumule is released.
Another method of penetrating the soil is found in the corn and in general by those plants whose first leaves are long and slender. In these cases protection is secured by the leaves being tightly rolled into a point and covered by a cap, so that they pierce the soil directly, thus meeting less resistance and securing safety.
The lifting force of germinating seeds is seldom noticed, but is very great. Masses of earth a hundred times their weight are lifted by our tiny garden seedlings as they come up, forcing their way through the hardest soil.
The last and most important step in germination is the establishment of the young plant in its new environment. In describing this process it is necessary to treat of the development of each part of the embryo by itself.
The hypocotyl first penetrates the testa. Protected by its root cap and directed downward by gravitation, it begins at once the production of the primary root from its lower end. From this, in turn, the whole root system rapidly develops. The only region of growth is just back of the tip, which, protected by the cap, is safely pushed downward into the earth.
The cotyledons, as before explained, may rise above ground if the hypocotyl lengthens upward, or, if not, may remain below. In either case they act as a storage of food for the seedling.
The development of the plumule usually attracts most attention for from it arise the leaves, stem, and, later, the flowers and fruit. It constitutes the shoot of the plant.
The first organ to develop in germination is the root, because the function required by the seedling is the absorption which the root performs. Many of the statements made in this, and the preceding chapter, can be proven by simple experiments.
There are a few plants whose seed will develop under water while others retain enough of the scant dews of the desert nights to waken the seed into growth.
Usually, however, a moderate water supply is essential, too much causing decay, and too little precluding growth altogether. As to temperature, a maple seedling will germinate on a cake of ice and many other seeds grow in extreme cold, while a smaller number tolerate high temperatures.
The majority, however, germinate most freely between 60° and 80° F. Air from some source is essential to growth, for seeds, like all living things, must breathe. Many can obtain the needed supply even from the air dissolved in the water in which they maybe submerged.
In addition to these external conditions, the embryo must also have a supply of stored food for immediate use while the roots .and leaves are developing. This food may be stored in the cotyledons, as in the bean and pea, or outside the embryo, as in the case of the endosperm of the corn and other grains.
The seed is not a thing totally distinct from the parent plant, though it is separated from it It contains the same protoplasm as the parent plant, with this distinction; its protoplasm is in a condition of rest The seed is not dead, it is asleep and waits only for favorable conditions to wake into the activity of growth.
This resting stage is of two-fold value — it condenses the essential nature of the whole plant within small compass, capable of easy and wide dispersal, and, most important of all, protects the vitality of the embryo so that the seed can withstand periods of drought, cold, heat, or other conditions which would be fatal to the parent plant. Both dispersal and preservation are steps toward the chief function of the seed, which is to reproduce the plant that is at rest within it. This resumption of active life is called germination
Endosperm. Though the endosperm is usually present at some stage, it is not found in all seeds when they are mature, since it may be entirely absorbed by the growing embryo, its function of food storage being assumed by the cotyledons. It is, however, very important in many seeds, especially the grains. From its store of starch we derive our bread. Food for the embryo may be stored either in the endosperm or cotyledons. Our laboratory tests show that this stored food consists largely of starch, together with considerable proteid, a little fat or oil, and some mineral matter.
The seed has within itself the miniature plant, or embryo, and all the kinds of nutrients needed for growth except water. This the seed must get from the soil before it can grow. The growth of a seed is a very wonderful process. Though inactive, dry, and apparently dead the protoplasm is really alive and only awaits favorable conditions for growth to begin.
The insoluble, stored foods must be digested by the embryo, made soluble, united with the water which has been absorbed from the soil, and assimilated, to form all the new kinds of tissue in the growing seedling. It may seem strange to speak of a seed as digesting food, but there is a substance (diastase) in the seed, which digests its food just as truly as the fluids of our stomach digest ours. Here, then, are digestion, absorption, and assimilation going on in the seed as it begins to grow. If the food stuffs in the seed were not stored in a dry and insoluble form, they would dissolve and decay. It is necessary, therefore, if a seed is to keep over winter, that its food must be both dry and insoluble.
Hypocotyl
The primitive stem, or all that part of the embryo below the cotyledons, is the hypocotyl. From its lower end the root system develops. Upon its upward lengthening depends whether the cotyledons shall emerge from the soil when germination takes place.
The plumule is that part of the embryo above the cotyledons, from which develops the shoot proper, consisting of stem, leaves, and flowers. It may vary much in size and development. If much food be stored, either in cotyledons or endosperm, the plumule may be small.
On the other hand if little food be provided, the plant must early shift for itself, and so the plumule may have several well-formed leaves, wanting only exposure to light to become a self supporting plant.
Fats. The last class of nutrients is the fats and oils, which are also composed of carbon, hydrogen, and oxygen. They differ from carbohydrates in having less oxygen. Hence they oxidize more readily and as a result their chief use is to produce energy.
Plants store fats in their seeds to supply energy for growth; animals store fats in various places and use them for the same purpose.
Kinds. Cotton-seed oil, olive oil, and the oils from various nuts are examples of vegetable fats; while lard, butter, and fat meats are familiar examples of fat from animals.
The substance should be crushed as finely as possible and treated with ether. This will dissolve out any fat or oil that may be present and the solution can then be poured off. When the ether evaporates the fat will remain in the dish.
Carbohydrates. Next to proteids in importance to all living things come the carbohydrates. They are composed of carbon, hydrogen, and oxygen, with always twice as much hydrogen as oxygen, and varying amounts of carbon. Carbohydrates are found almost entirely in plants, whose tissues they largely compose. When animals eat them, they usually oxidize them as fuel to produce heat and energy. Some are converted into fats and stored as such.
Some common carbohydrates are:
Complicated forms found in wood, paper, cotton, linen. (Glycogen is an animal carbohydrate found in the liver of some animals and called " liver starch." It seems to be stored there for later use.)
It is a little strange to think of cotton and starch, or wood and sugar as being so nearly related, but they consist of the same three elements, and are produced by the plants from water and carbon dioxide. It would be a cheap diet, if we could take water from a reservoir and carbon dioxide from the air and make them into flour. Man has to depend on plants for this wonderful process, and can only begin where the plants leave off, using the plantmade carbohydrates for his food.
The Test for Starches.
No one test can be used for all the carbohydrates, but we can test for any starch by dissolving the substance supposed to contain it in hot water and then adding a drop of iodine. The solution will turn blue if starch be present. No substance other than starch will act this way under these conditions.
The Test for Grape Sugar.
There is no one test for all sugars, but grape sugar (glucose) is very common and can be easily distinguished from our household (beet or cane) sugar by what is known as the Fettling Test — so named from the man who devised it. Two solutions are used in the Fehling test, one colorless, and one blue. When these are added in equal amounts to a similar amount of the substance to be tested, and the mixture heated, a yellowbrown solid will form if grape sugar be present. Cane or beet sugar will not act this way.
Proteids.
These are very numerous and are found in all living substances; the following are some that are common and found in large amounts.
It is not necessary to learn these names but the list is put in to show that proteids are of many kinds and, though first provided by plants, are needed in animal tissue as well.
Test for Proteids. Proteids differ in many ways but there is one point in which they all behave alike and which is different from any other substance —hence we can use it as a test If a substance supposed to contain any proteid is put into nitric acid and heated gently, it will turn bright yellow.
Then if the acid be washed off and ammonia added the proteid, if present, will become orange color. This is the test for any proteid for no other substance will act in the same way.
The proteids are the most useful of the nutrients for they make up most of the active living substance of plant and animal; they are called tissue builders on this account. Proteids are composed of the elements carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, with sometimes mineral salts as well, so we see they are very complex organic compounds.
