CHAPTER VIII.

STARCH AND SUGAR CONCLUDED. ANALYSIS OF PLANT SUBSTANCE.

1. Translocation of Starch.

=152. Translocation of starch.=—It has been found that leaves of many plants grown in the sunlight contain starch when examined after being in the sunlight for several hours. But when the plants are left in the dark for a day or two the leaves contain no starch, or a much smaller amount. This suggests that starch after it has been formed may be transferred from the leaves, or from those areas of the leaves where it has been formed.

To test this let us perform an experiment which is often made. We may take a plant such as a garden tropæolum or a clover plant, or other land plant in which it is easy to test for the presence of starch. Pin a piece of circular cork, which is smaller than the area of the leaf, on either side of the leaf, as in fig. 72, but allow free circulation of air between the cork and the under side of the leaf. Place the plant where it will be in the sunlight. On the afternoon of the following day, if the sun has been shining, test the entire leaf for starch. The part covered by the cork will not give the reaction for starch, as shown by the absence of the bluish color, while the other parts of the leaf will show it. The starch which was in that part of the leaf the day before was dissolved and removed during the night, and then during the following day, the parts being covered from the light, no starch was formed in them.

=153. Starch in other parts of plants than the leaves.=—We may use the iodine test to search for starch in other parts of plants than the leaves. If we cut a potato tuber, scrape some of the cut surface into a pulp, and apply the iodine test, we obtain a beautiful and distinct reaction showing the presence of starch. Now we have learned that starch is only formed in the parts containing chlorophyll. We have also learned that the starch which has been formed in the leaves disappears from the leaf or is transferred from the leaf. We judge therefore that the starch which we have found in the tuber of the potato was formed first in the green leaves of the plant, as a result of photosynthesis. From the leaves it is transferred in solution to the underground stems, and stored in the tubers. The starch is stored here by the plant to provide food for the growth of new plants from the tubers, which are thus much more vigorous than the plants would be if grown from the seed.

=154. Form of starch grains.=—Where starch is stored as a reserve material it occurs in grains which usually have certain characters peculiar to the species of plant in which they are found. They vary in size in many different plants, and to some extent in form also. If we scrape some of the cut surface of the potato tuber into a pulp and mount a small quantity in water, or make a thin section for microscopic examination, we find large starch grains of a beautiful structure. The grains are oval in form and more or less irregular in outline. But the striking peculiarity is the presence of what seem to be alternating dark and light lines in the starch grain. We note that the lines form irregular rings, which are smaller and smaller until we come to the small central spot termed the “hilum” of the starch grain. It is supposed that these apparent lines in the starch grain are caused by the starch substance being deposited in alternating dense and dilute layers, the dilute layers containing more water than the dense ones; others think that the successive layers from the hilum outward are regularly of diminishing density, and that this gives the appearance of alternating lines. The starch formed by plants is one of the organic substances which are manufactured by plants, and it (or glucose) is the basis for the formation of other organic substances in the plant. Without such organic substances green plants cannot make any appreciable increase of plant substance, though a considerable increase in size of the plant may take place.

NOTE.—The organic compounds resulting from photosynthesis, since they are formed by the union of carbon, hydrogen, and oxygen in such a way that the hydrogen and oxygen are usually present in the same proportion as in water, are called _carbohydrates_. The most common carbohydrates are sugars (cane sugar, C₁₂H₂₂O₁₁ for example, in beet roots, sugar cane, sugar maple, etc.), starch, and cellulose.

=155. Vaucheria.=—The result of carbon dioxide assimilation in the threads of Vaucheria is not clearly understood. Starch is absent or difficult to find in all except a few species, while oil globules are present in most species. These oil globules are spherical, colorless, globose and highly refringent. Often small ones are seen lying against chlorophyll bodies. Oil is a _hydrocarbon_ (containing C, H, and O, but the H and O are in different proportions from what they are in H₂O) and until recently it was supposed that this oil in Vaucheria was the direct result of photosynthesis. But the oil does not disappear when the plant is kept for a long time in the dark, which seems to show that it is not the direct product of carbon dioxide assimilation, and indicates that it comes either from a temporary starch body or from glucose. Schimper found glucose in several species of Vaucheria, and Waltz says that some starch is present in Vaucheria sericea, while in V. tuberosa starch is abundant and replaces the oil. To test for oil bodies in Vaucheria treat the threads with weak osmic acid, or allow them to stand for twenty-four hours in Fleming’s solution (which contains osmic acid). Mount some threads and examine with microscope. The oil globules are stained black.

2. Sugar, and Digestion of Starch.[9]

=156.= It is probable that some form of sugar is always produced as the result of photosynthesis. The sugar thus formed may be stored as such or changed to starch. In general it may be said that sugar is most common in the green parts of monocotyledonous plants, while starch is most frequent in dicotyledons. Plant sugars are of three general kinds: cane sugar abundant in the sugar cane, sugar beet, sugar maple, etc.; glucose and fruit sugar, found in the fruits of a majority of plants, and abundant in some, as in apples, pears, grapes, etc.; and maltose, a variety produced in germinating seeds, as in malted barley.

=157. Test for sugar.=—A very pretty experiment maybe made by taking two test tubes, placing in one a solution of commercial grape sugar (glucose), in the other one of granulated cane sugar, and adding to each a few drops of Fehling’s solution.[10] After these tubes have stood in a warm place for half an hour, it will be found that a bright orange brown or cinnabar-colored precipitate of copper and cuprous oxide has formed in the tube containing grape sugar, while the other solution is unchanged. Grape sugar or glucose, therefore, reduces Fehling’s solution, while cane sugar as such has no effect upon it.

Cane sugar may be changed or converted to glucose by being boiled for a short time with a dilute acid, or by adding Fehling’s solution to the sugar solution and boiling. In the latter case the change is brought about by the alkali and the precipitate of copper and cuprous oxide forms.

=158. Tests for sugar in plant tissue.=—(_a_) Scrape out a little of the tissue from the inside of a ripe apple or pear, place it with a little water in a test tube, and add a few drops of Fehling’s solution. After standing half an hour the characteristic precipitate of copper and cuprous oxide appears, showing that grape sugar is present in quantity.

Make thin sections of the apple and mount in a drop of Fehling’s solution on a slide. After half an hour examine with the microscope. The granules of cuprous oxide are present in the cells of the tissue in great abundance.

(_b_) Cut up several leaves of a young vigorous corn seedling, cover with water in a test tube and boil for a minute. After the decoction has cooled add the Fehling’s solution and allow to stand. The precipitate will appear. For comparison take similar corn leaves, remove the chlorophyll with alcohol and test with iodine. No starch reaction appears. The carbohydrate in corn leaves is therefore glucose and not starch. If now the corn seed be examined the cells will be found to be full of starch grains which give the beautiful blue reaction with iodine. This experiment shows that grape sugar is formed in the leaves of the corn plant, but is changed to starch when stored in the seed.

(_c_) Take two leaves of bean seedling or coleus, test one for sugar and the other for starch. Both are present.

(_d_) Procure some maple sap in the spring, or in the winter months make a decoction of the broken tips of young branches of the sugar maple by boiling them in water in a test tube. To the sap or cool decoction add Fehling’s solution. No precipitate appears after standing. Now heat the same solution to the boiling point, and the precipitate forms, showing the presence of cane sugar in the maple sap which was converted to glucose and fruit sugar by boiling in the presence of an alkali.

(_e_) Scrape out some of the tissue from a sugar beet root, cover with water in a test tube and add Fehling’s solution. No change takes place after standing. Boil the same solution and the precipitate forms, showing the presence of cane sugar, inverted to grape sugar and fruit sugar by the hot alkali.

=159. How starch is changed to sugar.=—We have seen that in many plants the carbohydrate formed as the result of carbon dioxide assimilation is stored as starch. This substance being insoluble in water must be changed to sugar, which is soluble before it can be used as food or transported to other parts of the plant. This is accomplished through the action of certain enzymes, principally diastase. This substance has the power of acting upon starch under proper conditions of temperature and moisture, causing it to take up the elements of water, and so to become sugar.

This process takes place commonly in the leaves where starch is formed, but especially in seeds, tubers (during the sprouting, etc.), and other parts which the plant uses as storehouses for starch food. It is probable that the same conditions of temperature and moisture which favor germination or active growth are also favorable to the production of diastase.

=160. Experiments to show the action of diastase.=—(_a_) Place a bit of starch half as large as a pea in a test tube, and cover with a weak solution[11] (about ⅕ per cent) of commercial taka diastase. After it has stood in a warm place for five or ten minutes test with Fehling’s solution. The precipitate of cuprous oxide appears showing that some of the starch has been changed to sugar. By using measured quantities, and by testing with iodine at frequent intervals, it can be determined just how long it takes a given quantity of diastase to change a known quantity of starch. In this connection one should first test a portion of the same starch with Fehling’s solution to show that no sugar is present.

(_b_) Repeat the above experiment using a little tissue from a potato, and some from a corn seed.

(_c_) Take 25 germinating barley seeds in which the radicle is just appearing. Grind up thoroughly in a mortar with about three parts of water. After this has stood for ten or fifteen minutes, filter. Fill a test tube one-third full of water, add a piece of starch half the size of a pea or less, and boil the mixture to make starch-paste. Add the barley extract. Put in a warm place and test from time to time with iodine. The first samples so treated will be blue, later ones violet, brown, and finally colorless, showing that the starch has all disappeared. This is due to the action of the diastase which was present in the germinating seeds, and which was dissolved out and added to the starch mixture. The office of this diastase is to change the starch in the seeds to sugar. Germinating wheat is sweet, and it is a matter of common observation that bread made from sprouted wheat is sweet.

(_d_) Put a little starch-paste in a test tube and cover it with saliva from the mouth. After ten or fifteen minutes test with Fehling’s solution. A strong reaction appears showing how quickly and effectively saliva acts in converting starch to sugar. Successive tests with iodine will show the gradual disappearance of the starch.

=161. These experiments have shown us that diastase= from three different sources can act upon starch converting it into sugar. The active principle in the saliva is an _animal_ diastase (_ptyalin_), which is necessary as one step in the digestion of starch food in animals. The _taka_ diastase is derived from a fungus (Eurotium oryzæ) which feeds on the starch in rice grains converting it into sugar which the fungus absorbs for food. The _malt_ diastase and _leaf_ diastase are formed by the seed plants. That in seeds converts the starch to sugar which is absorbed by the embryo for food. That in the leaf converts the starch into sugar so that it can be transported to other parts of the plant to be used in building new tissue, or to be stored again in the form of starch (example, the potato, in seeds, etc.). The starch is formed in the leaf during the daylight. The light renders the leaf diastase inactive. But at night the leaf diastase becomes active and converts the starch made during the day. Starch is not soluble in water, while the sugar is, and the sugar in solution is thus easily transported throughout the plant. In those green plants which do not form starch in their leaves (sugar beet, corn, and many monocotyledons), grape sugar and fruit sugar are formed in the green parts as the result of photosynthesis. In some, like the corn, the grape sugar formed in the leaves is transported to other parts of the plant, and some of it is stored up in the seed as starch. In others like the sugar beet the glucose and fruit sugar formed in the leaves flow to other parts of the plant, and much of it is stored up as cane sugar in the beet root. The process of photosynthesis probably proceeds in the same way in all cases up to the formation of the grape sugar and fruit sugar in the leaves. In the beet, corn, etc., the process stops here, while in the bean, clover, and most dicotyledons the process is carried one step farther in the leaf and starch is formed.

3. Rough Analysis of Plant Substance.

=162. Some simple experiments to indicate the nature of plant substance.=—After these building-up processes of the plant, it is instructive to perform some simple experiments which indicate roughly the nature of the plant substance, and serve to show how it can be separated into other substances, some of them being reduced to the form in which they existed when the plant took them as food. For exact experiments and results it would be necessary to make chemical analyses.

=163. The water in the plant.=—Take fresh leaves or leafy shoots or other fresh plant parts. Weigh. Permit them to remain in a dry room until they are what we call “dry.” Now weigh. The plants have lost weight, and from what we have learned in studies of transpiration this loss in weight we know to result from the loss of water from the plant.

=164. The dry plant material contains water.=—Take air-dry leaves, shavings, or other dry parts of plants. Place them in a test tube. With a holder rest the tube in a nearly horizontal position, with the bottom of the tube in the flame of a Bunsen burner. Very soon, before the plant parts begin to “burn,” note that moisture is accumulating on the inner surface of the test tube. This is water driven off which could not escape by drying in air, without the addition of artificial heat, and is called “hygroscopic water.”

=165. Water formed on burning the dry plant material.=—Light a soft-pine or basswood splinter. Hold a thistle tube in one hand with the bulb downward and above the flame of the splinter. Carbon will be deposited over the inner surface of the bulb. After a time hold the tube toward the window and look through it above the carbon. Drops of water have accumulated on the inside of the tube. This water is formed by the rearrangement of some of the hydrogen and oxygen, which is set free by the burning of the plant material, where they were combined with carbon, as in the cellulose, and with other elements.

=166. Formation of charcoal by burning.=—Take dried leaves, and shavings from some soft wood. Place in a porcelain crucible, and cover about 3 cm. deep with dry fine earth. Place the crucible in the flame of a Bunsen burner and let it remain for about fifteen minutes. Remove and empty the contents. If the flame was hot the plant material will be reduced to a good quality of charcoal. The charcoal consists largely of carbon.

=167. The ash of the plant.=—Place in the porcelain crucible dried leaves and shavings as before. Do not cover with earth. Place the crucible in the flame of the Bunsen burner, and for a moment place on the porcelain cover; then remove the cover, and note the moisture on the under surface from the escaping water. Permit the plant material to burn; it may even flame for a time. In the course of fifteen minutes it is reduced to a whitish powder, much smaller in bulk than the charcoal in the former experiment. This is the ash of the plant.

=168. What has become of the carbon?=—In this experiment the air was not excluded from the plant material, so that oxygen combined with carbon as the water was freed, and formed carbon dioxide, passing off into the air in this form. This it will be remembered is the form in which the plant took the carbon-food in through the leaves. Here the carbon dioxide met the water coming from the soil, and the two united to form, ultimately, starch, cellulose, and other compounds of carbon; while with the addition of nitrogen, sulphur, etc., coming also from the soil, still other plant substances were formed.

=169.= The carbohydrates are classed among the non-nitrogenous substances. Other non-nitrogenous plant substances are the organic acids like oxalic acid (H₂C₂O₄), malic acid (H₂C₄H₄O₅), etc.; the fats and fixed oils, which occur in the seeds and fruits of many plants. Of the nitrogenous substances the proteids have a very complex chemical formula and contain carbon, hydrogen, oxygen, nitrogen, sulphur, etc. (example, _aleuron_, or proteid grains, found in seeds). The proteids are the source of nitrogenous food for the seedling during germination. Of the amides, _asparagin_ (C₄H₈N₂O₃) is an example of a nitrogenous substance; and of the alkaloids, nicotin (C₁₀H₁₄N₂) from tobacco.

All living plants contain a large per cent of water. According to Vines “ripe seeds dried in the air contain 12 to 15 per cent of water, herbaceous plants 60 to 80 per cent, and many water plants and fungi as much as 95 per cent of their weight.” When heated to 100° C. the water is driven off. The dry matter remaining is made up partly of organic compounds, examples of which are given above, and inorganic compounds. By burning this dry residue the organic substances are mostly changed into volatile products, principally carbonic acid, water, and nitrogen. The inorganic substances as a result of combustion remain as a white or gray powder, the _ash_.

The amount of the ash increases with the age of the plant, though the percentage of ash may vary at different times in the different members of the plant. The following table taken from Vines will give an idea of the amount and composition of the ash in the dry solid of a few plants:

CONTENT OF 1000 PARTS OF DRY SOLID MATTER.

|Clover, in|Wheat,|Wheat,|Potato|Apples| Peas | blossom |grain |straw |tubers| |(the seed) ------------------+----------+------+------+------+------+---------- Ash. | 68.3 | 19.7 | 53.7 | 37.7 | 14.4 | 27.3 Potash. | 21.96 | 6.14 | 7.33 | 22.76| 5.14| 11.41 Soda. | 1.39 | 0.44 | 0.74 | 0.99| 3.76| 0.26 Lime. | 24.06 | 0.66 | 3.09 | 0.97| 0.59| 1.36 Magnesium. | 7.44 | 2.36 | 1.33 | 1.77| 1.26| 2.17 Ferric Oxide. | 0.72 | 0.26 | 0.33 | 0.45| 0.2 | 0.16 Phosphoric Acid. | 6.74 | 9.26 | 2.58 | 6.53| 1.96| 9.95 Sulphuric Acid. | 2.06 | 0.07 | 1.32 | 2.45| 0.88| 0.95 Silica. | 1.62 | 0.42 |36.25 | 0.8 | 0.62| 0.24 Chlorine. | 2.66 | 0.04 | 0.9 | 1.17| ....| 0.42 ------------------+----------+------+------+------+------+----------

FOOTNOTES:

[9] Paragraphs 156-160 were prepared by Dr. E. J. Durand.

[10] Make up three stock solutions as follows: (1) Copper sulphate 9 grams Water 250 cc. (2) Caustic potash 30 grams Water 250 cc. (3) Rochelle salts 49 grams Water 250 cc. For Fehling’s solution take one volume of each of (1), (2), and (3), and to the mixture add two volumes of water.

[11] This solution of taka diastase should be made up cold. If it is heated to 60° C. or over it is destroyed.