CHAPTER XIII

LEAVES--FUNCTION OR WORK

We have discussed (in Chap. VIII) the work or function of roots and also (in Chap. X) the function of stems. We are now ready to complete the view of the main vital activities of plants by considering the function of the green parts (leaves and young shoots).

=Sources of Food.=--The ordinary green plant has but _two sources from which to secure food,--the air and the soil_. When a plant is thoroughly dried in an oven, the water passes off; _this water came from the soil_. The remaining part is called the =dry substance or dry matter=. If the dry matter is burned in an ordinary fire, only the =ash= remains; _this ash came from the soil_. The part that passed off as gas in the burning _contained the elements that came from the air_; it also contained some of those that came from the soil--all those (as nitrogen, hydrogen, chlorine) that are transformed into gases by the heat of a common fire. The part that comes from the soil (the ash) is small in amount, being considerably less than 10 per cent and sometimes less than 1 per cent. _Water is the most abundant single constituent or substance of plants._ In a corn plant of the roasting-ear stage, about 80 per cent of the substance is water. A fresh turnip is over 90 per cent water. Fresh wood of the apple tree contains about 45 per cent of water.

=Carbon.=--_Carbon enters abundantly into the composition of all plants._ Note what happens when a plant is burned without free access of air, or smothered, as in a charcoal pit. _A mass of charcoal remains, almost as large as the body of the plant._ Charcoal is almost pure _carbon_, the ash present being so small in proportion to the large amount of carbon that we look on the ash as an impurity. Nearly half of the dry substance of a tree is carbon. Carbon goes off as _a gas_ when the plant is _burned in air_. It does not go off alone, but in combination with oxygen in the form of _carbon dioxid gas_, CO₂.

=The green plant secures its carbon from the air.= In other words, much of the _solid matter_ of the plant comes from _one of the gases of the air_. By volume, _carbon dioxid forms only a very small fraction of 1 per cent of the air_. It would be very disastrous to animal life, however, if this percentage were much increased, for it excludes the life-giving oxygen. Carbon dioxid is often called “foul gas.” It may accumulate in old wells, and an experienced person will not descend into such wells until they have been tested with a torch. If the air in the well will not support combustion,--that is, if the torch is extinguished,--it usually means that carbon dioxid has drained into the place. The air of a closed schoolroom often contains far too much of this gas, along with little solid particles of waste matters. Carbon dioxid is often known as carbonic acid gas.

=Appropriation of the Carbon.=--_The carbon dioxid of the air readily diffuses itself into the leaves and other green parts of the plant._ The leaf is delicate in texture, and when very young the air can diffuse directly into the tissues. The stomates, however, are the special inlets adapted for the admission of gases into the leaves and other green parts. Through these stomates, or diffusion-pores, the outside air enters into the air-spaces of the plant, and is finally absorbed by the little cells containing the living matter.

=Chlorophyll= (“leaf green”) is the agent that secures the energy by means of which carbon dioxid is utilized. This material is contained in the leaf cells in the form of grains (p. 86); the grains themselves are protoplasm, only the coloring matter being chlorophyll. _The chlorophyll bodies or grains are often most abundant near the upper surface of the leaf, where they can secure the greatest amount of light._ Without this green coloring matter, there would be no reason for the large flat surfaces which the leaves possess, and no reason for the fact that the leaves are borne most abundantly at the ends of branches, where the light is most available. Plants with colored leaves, as coleus, have chlorophyll, but it is masked by other coloring matter. This other coloring matter is usually soluble in hot water: boil a coleus leaf and notice that it becomes green and the water becomes colored.

_Plants grown in darkness are yellow and slender, and do not reach maturity._ Compare the potato sprouts that have grown from a tuber lying in the dark cellar with those that have grown normally in the bright light. The shoots have become slender and are devoid of chlorophyll; and when the food that is stored in the tuber is exhausted, these shoots will have lived useless lives. A plant that has been grown in darkness from the seed will soon die, although for a time the little seedling will grow very tall and slender: why? _Light favors the production of chlorophyll_, and the chlorophyll is the agent in the making of _the organic carbon compounds_. Sometimes chlorophyll is found in buds and seeds, but in most cases these places are not perfectly dark. Notice how potato tubers develop chlorophyll, or become green, when exposed to light.

=Photosynthesis.=--_Carbon dioxid diffuses into the leaf; during sunlight it is used, and oxygen is given off._ How the carbon dioxid which is thus absorbed may be used in making an organic food is a complex question, and need not be studied here; but it may be stated that carbon dioxid and water are the constituents. Complex compounds are built up out of simpler ones.

_Chlorophyll absorbs certain light rays, and the energy thus directly or indirectly obtained is used by the living matter in uniting the carbon dioxid absorbed from the air with some of the water brought up from the roots. The ultimate result usually is starch._ The process is obscure, but sugar is generally one step; and our first definite knowledge of the product begins when starch is deposited in the leaves. The process of using the carbon dioxid of the air has been known as carbon assimilation, but the term now most used is =photosynthesis= (from two Greek words, meaning _light and to put together_).

=Starch and Sugar.=--_All starch is composed of carbon, hydrogen, and oxygen_ (C₆H₁₀O₅)_ₙ_. The sugars and the substance of cell walls are very similar to it in composition. All these substances are called =carbohydrates=. In making fruit sugar from the carbon and oxygen of carbon dioxid and from the hydrogen and oxygen of the water, _there is a surplus of oxygen_ (6 parts CO₂ + 6 parts H₂O = C₆H₁₂O₆ + 6 O₂). It is this oxygen that is given off into the air during sunlight.

=Digestion.=--_Starch is in the form of insoluble granules. When such food material is carried from one part of the plant to another for purposes of growth or storage, it is made soluble before it can be transported._ When this starchy material is transferred from place to place, it is usually changed into sugar by the action of a diastase. _This is a process of_ =digestion=. It is much like the change of starchy foodstuffs to sugary foods by the saliva.

=Distribution of the Digested Food.=--After being changed to the soluble form, _this material is ready to be used in growth_, either in the leaf, in the stem, or in the roots. With other more complex products it is then _distributed throughout all of the growing parts of the plant;_ and when passing down to the root, it seems to pass more readily through the _inner bark_, in plants which have a definite bark. This gradual downward diffusion through the inner bark of materials suitable for growth is the process referred to when the “descent of sap” is mentioned. Starch and other products are often _stored in one growing season to be used in the next season_. If a tree is constricted or strangled by a wire around its trunk (Fig. 118), the digested food cannot readily pass down and it is stored above the girdle, causing an enlargement.

[Illustration: FIG. 118.--TRUNK GIRDLED BY A WIRE. See Fig. 85.]

=Assimilation.=--_The food from the air and that from the soil unite in the living tissues._ The “sap” that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. 67). This upward-moving water is conducted largely through certain tubular canals of the _young wood_. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course.

The upward-moving water gradually passes to the growing parts, and everywhere in the living tissues, it is of course in the most intimate contact with the soluble carbohydrates and products of photosynthesis. In the building up or reconstructive and other processes it is therefore available. We may properly conceive of certain of the simpler organic molecules as passing through a series of changes, gradually increasing in complexity. There will be formed substances containing nitrogen in addition to carbon, hydrogen, and oxygen. Others will contain also sulfur and phosphorus, and the various processes may be thought of as culminating in =protoplasm=. _Protoplasm is the living matter in plants._ It is in the cells, and is usually semifluid. Starch is not living matter. The complex process of building up the protoplasm is called =assimilation=.

=Respiration.=--_Plants need oxygen for respiration, as animals do._ We have seen that plants need the carbon dioxid of the air. To most plants the nitrogen of the air is inert, and serves only to dilute the other elements; but the _oxygen is necessary for all life_. We know that all animals need this oxygen in order to breathe or respire. In fact, they have become accustomed to it in just the proportions found in the air; and this is now best for them. When animals breathe the air once, they make it foul, because they use some of the oxygen and give off carbon dioxid. Likewise, _all living parts of the plant must have a constant supply of oxygen_. Roots also need it, for they respire. Air goes in and out of the soil by diffusion, and as the soil is heated and cooled, causing the air to expand and contract.

The oxygen passes into the air-spaces and is absorbed by the moist cell membranes. In the living cells it makes possible the formation of simpler compounds by which energy is released. This energy enables the plant to work and grow, and the final products of this action are _carbon dioxid and water_. As a result of the use of this oxygen by night and by day, plants give off carbon dioxid. _Plants respire; but since they are stationary, and more or less inactive, they do not need as much oxygen as animals, and they do not give off so much carbon dioxid._ A few plants in a sleeping room need not disturb one more than a family of mice. It should be noted, however, that germinating seeds respire vigorously, hence they consume much oxygen; and opening buds and flowers are likewise active.

=Transpiration.=--Much more water is absorbed by the roots than is used in growth, _and this surplus water passes from the leaves into the atmosphere by an evaporation process known as_ =transpiration=. Transpiration takes place more abundantly from the under surfaces of leaves, and through the pores or stomates. A sunflower plant of the height of a man, during an active period of growth, gives off a quart of water per day. A large oak tree may transpire 150 gallons per day during the summer. For every ounce of dry matter produced, it is estimated that 15 to 25 pounds of water usually passes through the plant.

_When the roots fail to supply to the plant sufficient water to equalize that transpired by the leaves_, =the plant wilts=. Transpiration from the leaves and delicate shoots is increased by all of the conditions which increase evaporation, such as higher temperature, dry air, or wind. The stomata open and close, tending to regulate transpiration as the varying conditions of the atmosphere affect the moisture content of the plant. However, in periods of drought or of very hot weather, and especially during a hot wind, the closing of these stomates cannot sufficiently prevent evaporation. The roots may be very active and yet fail to absorb sufficient moisture to equalize that given off by the leaves. The plant shows the effect (how?). On a hot dry day, note how the leaves of corn “roll” towards afternoon. Note how fresh and vigorous the same leaves appear early the following morning. Any injury to the roots, such as a bruise, or exposure to heat, drought, or cold may cause the plant to wilt.

Water is forced up by =root pressure= or =sap pressure=. (Exercise 99.) Some of the dew on the grass in the morning may be the water forced up by the roots; some of it is the condensed vapor of the air.

_The wilting of a plant is due to the loss of water from the cells._ The cell walls are soft, and collapse. A toy balloon will not stand alone until it is inflated with air or liquid. In the woody parts of the plant the cell walls may be stiff enough to support themselves, even though the cell is empty. Measure the contraction due to wilting and drying by tracing a fresh leaf on page of notebook, and then tracing the same leaf after it has been dried between papers. The softer the leaf, the greater will be the contraction.

=Storage.=--We have said that starch may be stored in twigs to be used the following year. The very early flowers on fruit trees, especially those that come before the leaves, and those that come from bulbs, as crocuses and tulips, are supported by the starch or other food that was organized the year before. Some plants have very special storage reservoirs, as the potato, in this case being a thickened stem although growing underground. (Why a thickened stem? p. 84.) It is well to make the starch test on winter twigs and on all kinds of thickened parts, as tubers and bulbs.

=Carnivorous Plants.=--Certain plants capture insects and other very small animals and utilize them to some extent as food. Such are the sundew, that has on the leaves sticky hairs that close over the insect; the Venus’s flytrap of the Southern states, in which the halves of the leaves close over the prey like the jaws of a steel trap; and the various kinds of pitcher plants that collect insects and other organic matter in deep, water-filled, flask-like leaf pouches (Fig. 119).

The sundew and Venus’s flytrap are sensitive to contact. Other plants are _sensitive to the touch_ without being insectivorous. The common cultivated sensitive plant is an example. This is readily grown from seeds (sold by seedsmen) in a warm place. Related wild plants in the south are sensitive. The utility of this sensitiveness is not understood.

[Illustration: FIG. 119.--THE COMMON PITCHER PLANT (_Sarracenia purpurea_) of the North, showing the tubular leaves and the odd, long-stalked flowers.]

=Parts that Simulate Leaves=.--We have learned that leaves are endlessly modified to suit the conditions in which the plant is placed. The most marked modifications are in adaptation to light. On the other hand, _other organs often perform the functions of leaves_. Green shoots function as leaves. These shoots may look like leaves, in which case they are called =cladophylla=. The foliage of common asparagus is made up of fine branches: the real morphological leaves are the minute dry functionless scales at the bases of these branchlets. (What reason is there for calling them leaves?) The broad “leaves” of the florist’s smilax are cladophylla: where are the leaves on this plant? In most of the cacti, the entire plant body performs the functions of leaves until the parts become cork-bound.

=Leaves are sometimes modified to perform other functions than the vital processes=: they may be tendrils, as the terminal leaflets of pea and sweet pea; or spines, as in barberry. Not all spines and thorns, however, represent modified leaves: some of them (as of hawthorns, osage orange, honey locust) are branches.

[Illustration: FIG. 120.--EXCLUDING LIGHT AND CO₂ FROM PART OF A LEAF]

[Illustration: FIG. 121.--THE RESULT.]

[Illustration: FIG. 122.--TO SHOW THE ESCAPE OF OXYGEN.]

[Illustration: FIG. 123.--TO ILLUSTRATE A PRODUCT OF RESPIRATION.]

[Illustration: FIG. 124.--RESPIRATION OF THICK ROOTS.]

[Illustration: FIG. 125.--TO ILLUSTRATE TRANSPIRATION.]

SUGGESTIONS.--_To test for chlorophyll._ =84.= Purchase about a gill of wood alcohol. Secure a leaf of geranium, clover, or other plant that has been exposed to sunlight for a few hours, and, after dipping it for a minute in boiling water, put it in a white cup with sufficient alcohol to cover. Place the cup in a shallow pan of hot water on the stove where it is not hot enough for the alcohol to take fire. After a time the chlorophyll is dissolved by the alcohol, which has become an intense green. Save this leaf for the starch experiment (Exercise 85). Without chlorophyll, the plant cannot appropriate the carbon dioxid of the air. _Starch and photosynthesis._ =85.= Starch is present in the green leaves which have been exposed to sunlight; but in the dark no starch can be formed from carbon dioxid. Apply iodine to the leaf from which the chlorophyll was dissolved in the previous experiment. Note that the leaf is colored purplish brown throughout. The leaf contains starch. =86.= Secure a leaf from a plant which has been in the darkness for about two days. Dissolve the chlorophyll as before, and attempt to stain this leaf with iodine. No purplish brown color is produced. This shows that the starch manufactured in the leaf may be entirely removed during darkness. =87.= Secure a plant which has been kept in darkness for twenty-four hours or more. Split a small cork and pin the two halves on opposite sides of one of the leaves, as shown in Fig. 120. Place the plant in the sunlight again. After a morning of bright sunshine dissolve the chlorophyll in this leaf with alcohol; then stain the leaf with the iodine. Notice that the leaf is stained deeply except where the cork was; there sunlight and carbon dioxid were excluded, Fig. 121. There is no starch in the covered area. =88.= Plants or parts of plants that have developed no chlorophyll can form no starch. Secure a variegated leaf of coleus, ribbon grass, geranium, or of any plant showing both white and green areas. On a day of bright sunshine, test one of these leaves by the alcohol and iodine method for the presence of starch. Observe that the parts devoid of green color have formed no starch. However, after starch has once been formed in the leaves, it may be changed into soluble substances and removed, to be again converted into starch in certain other parts of the living tissues. _To test the giving off of oxygen by day._ =89.= Make the experiment illustrated in Fig. 122. Under a funnel in a deep glass jar containing fresh spring or stream water place fresh pieces of the common waterweed elodea (or anacharis). Have the funnel considerably smaller than the vessel, and support the funnel well up from the bottom so that the plant can more readily get all of the carbon dioxid available in the water. Why would boiled water be undesirable in this experiment? For a home-made glass funnel, crack the bottom off a narrow-necked bottle by pressing a red-hot poker or iron rod against it and leading the crack around the bottle. Invert a test-tube over the stem of the funnel. In sunlight bubbles of oxygen will arise and collect in the test-tube. If a sufficient quantity of oxygen has collected, a lighted taper inserted in the tube will glow with a brighter flame, showing the presence of oxygen in greater quantity than in the air. Shade the vessel. Are bubbles given off? For many reasons it is impracticable to continue this experiment longer than a few hours. =90=. A simpler experiment may be made if one of the waterweeds Cabomba (water-lily family) is available. Tie a lot of branches together so that the basal ends shall make a small bundle. Place these in a large vessel of spring water, and insert a test-tube of water as before over the bundle. The bubbles will arise from the cut surfaces. Observe the bubbles on pond scum and waterweeds on a bright day. _To illustrate the results of respiration_ (CO₂). =91.= In a jar of germinating seeds (Fig. 123) place carefully a small dish of limewater and cover tightly. Put a similar dish in another jar of about the same air space. After a few hours compare the cloudiness or precipitate in the two vessels of limewater. =92.= Or, place a growing plant in a deep covered jar away from the light, and after a few hours insert a lighted candle or splinter. =93.= Or, perform a similar experiment with fresh roots of beets or turnips (Fig. 124) from which the leaves are mostly removed. In this case, the jar need not be kept dark; why? _To test transpiration._ =94.= Cut a succulent shoot of any plant, thrust the end of it through a hole in a cork, and stand it in a small bottle of water. Invert over this a fruit jar, and observe that a mist soon accumulates on the inside of the glass. In time drops of water form. =95.= The experiment may be varied as shown in Fig. 125. =96.= Or, invert the fruit jar over an entire plant, as shown in Fig. 126, taking care to cover the soil with oiled paper or rubber cloth to prevent evaporation from the soil. =97.= The test may also be made by placing the pot, properly protected, on balances, and the loss of weight will be noticed (Fig. 127). =98.= Cut a winter twig, seal the severed end with wax, and allow the twig to lie several days; it shrivels. There must be some upward movement of water even in winter, else plants would shrivel and die. =99.= _To illustrate sap pressure._ The upward movement of sap water often takes place under considerable force. The cause of this force, known as _root pressure_, is not well understood. The pressure varies with different plants and under different conditions. To illustrate: cut off a strong-growing small plant near the ground. By means of a bit of rubber tube attach a glass tube with a bore of approximately the diameter of the stem. Pour in a little water. Observe the rise of the water due to the pressure from below (Fig. 128). Some plants yield a large amount of water under a pressure sufficient to raise a column several feet; others force out little, but under considerable pressure (less easily demonstrated). _The vital processes_ (_i.e._, the life processes). =100.= The pupil having studied roots, stems, and leaves, should now be able to describe the main vital functions of plants: what is the root function? stem function? leaf function? =101.= What is meant by the “sap”? =102.= Where and how does the plant secure its water? oxygen? carbon? hydrogen? nitrogen? sulfur? potassium? calcium? iron? phosphorus? =103.= Where is all the starch in the world made? What does a starch-factory establishment do? Where are the real starch factories? =104.= In what part of the twenty-four hours do plants grow most rapidly in length? When is food formed and stored most rapidly? =105.= Why does corn or cotton turn yellow in a long rainy spell? =106.= If stubble, corn stalks, or cotton stalks are burned in the field, is as much plant-food returned to the soil as when they are plowed under? =107.= What process of plants is roughly analogous to perspiration of animals? =108.= What part of the organic world uses raw mineral for food? =109.= Why is earth banked over celery to blanch it? =110.= Is the amount of water transpired equal to the amount absorbed? =111.= Give some reasons why plants very close to a house may not thrive or may even die. =112.= Why are fruit-trees pruned or thinned out as in Fig. 129? _Proper balance between top and root._ =113.= We have learned that the leaf parts and the root parts work together. They may be said to balance each other in activities, the root supplying the top and the top supplying the root (how?). If half the roots were cut from a tree, we should expect to reduce the top also, particularly if the tree is being transplanted. How would you prune a tree or bush that is being transplanted? Fig. 130 may be suggestive.

[Illustration: FIG. 126.--TO ILLUSTRATE TRANSPIRATION.]

[Illustration: FIG. 127.--LOSS OF WATER.]

[Illustration: FIG. 128.--TO SHOW SAP PRESSURE.]

[Illustration: FIG. 129.--BEFORE AND AFTER PRUNING.]

[Illustration: FIG. 130.--AN APPLE TREE, with suggestions as to pruning when it is set in the orchard. At _a_ is shown a pruned top.]