Part II

of the former substances is not clear, but it is probable that the glycerin is transformed into some kind of sugar which travels into the tissues of the growing embryo where some of it is not unfrequently converted into a temporary reserve of small starch-grains. (vi) Another group of enzymes exists in plants by means of which the various insoluble and indiffusible proteins are hydro- lysed into simpler diffusible proteins, termed peptones, together with a larger or smaller amount of amides. So far as they have been examined they all resemble the enzyme secreted by the pancreas of the higher animals, and are termed vegetable trypsins. The chemical changes which proteins undergo in their migra- tion from place to place within the tissues of plants are not the same in all cases, but the reserve proteins of many seeds are made available for the embryo through the action of tryptic ferments. When germination begins the insoluble and slowly diffusible proteins in the cotyledons and endosperm are decom- posed into soluble peptones, and one or more amides, such as asparagine, leucine or tyrosine, all of which substances circulate readily to the various parts of the growing embryo needing nitrogenous nutriment. 232 ENZYMES Trypsins are also met with in the leaves, stems and developing fruits of many plants where they facilitate the rapid translocation of proteids in such organs. 3. The power which parasitic and saprophytic plants possess of absorbing and utilising as food the starch, proteins and various organic materials belonging to other plants, is dependent to a large extent upon their power of secreting diastatic and other enzymes. Certain parasitic fungi penetrate into the tissues of their victims by secreting an enzyme which is capable of dissolving the obstructing cell-walls. The production of alcohol from sugar by yeast is apparently effected by an enzyme named zymase, which is present in the yeast-cells, and some of the chemical changes brought about by bacteria are the result of the action of enzymes secreted by these organisms. -- Ex. 134. Germinate some barley grains on damp blotting-paper ; when the plumule just appears taste the endosperm and compare its sweetness with that of a soaked ungerminated grain. Compare the taste of malt with that of ordinary barley grains. -- Ex. 13B. Prepare some thin starch-paste and a solution of malt-diastase as described in Ex. 86. Take two tubes of starch-paste and into one pour some of the diastase- soldtion, and into the other some of the same solution after it has been boiled three minutes and then cooled. Test with iodine for starch in both tubes every five minutes as indicated in Ex. 86. What has been the effect of boiling the diastase solution ? CHAPTER XIX. RESPIRATION. Ordinary Eespiration in the presence of free oxygen of the -- atmosphere : aerobic respiration. One of the most familiar physiological processes carried on by living animals is that of respiration, during which there is a constant interchange of gases between the body of the animal and the surrounding air : the oxygen of the air is inspired into the lungs, and from the latter carbon dioxide gas is breathed out into the atmosphere. So long as life exists respiration goes on continuously, and one of the certain signs of death is the cessation of the process. Respiration, however, is not confined to animals, but is carried on by all ordinary plants, and is as necessary for their existence as for the existence of animals. The amount and rapidity of respiration is usually much greater in animals than in plants, but the process is essentially the same in both classes of organisms. It is well known that animals die when the supply of fresh air is cut off, and plants soon show signs of ill-health under similar conditions. In ordinary farm and garden practice the parts above ground always obtain sufficient oxygen for all their requirements, but the roots of plants are often seriously injured through want of a suitable supply of fresh air in the soil. The unhealthy appearance of over-watered pot plants and of crops growing in badly-drained ground is primarily due to an insufficient supply of oxygen to their roots. Seeds buried too deeply do not obtain sufficient fresh air for normal respiration and either do not germinate at all or do so in an unsatisfactory manner. "33 234 RESPIRATION Each living cell of the body of a plant respires, the oxygen necessary for the process being supplied from the air which penetrates through the stomata and lenticels and permeates throughout the plant in the intercellular spaces. In all the higher plants the products of respiration under normal conditions are carbon dioxide gas and water. As the carbon of the carbon dioxide is derived from the compounds within the body of the plant, it is clear that the process is a destructive one and must result in a decrease in the dry weight of the plant. The Seedlings of cereals and many other plants when allowed to grow in the dark often lose about half their dry substance in two or three weeks. In this respect respiration is essentially the opposite of the ' assimilation ' process in which there is a fixation of carbon and a consequent increase in dry weight of the plant. Moreover, respiration goes on in all living cells, both in darkness and in Ught, whereas ' carbon-fixation ' is only carried on by those cells which contain chloroplasts, and in these only when they are exposed to light. During respiration oxygen is consumed and carbon dioxide is set free into the air, but in green plants exposed to daylight the ' carbon-fixation ' process consumes twenty or thirty times as much carbon dioxide as is produced by respiration during the same time, so that when both processes are going on there is always a decrease in the carbon dioxide and an increase in the oxygen of the atmosphere, and only at night or in the dark does the process of respiration become apparent. However, in parts of plants which are not green, such as the roots, flowers and germinating seeds, respiration is readily detectable at all times. The carbon compounds which disappear while respiration is going on, are carbohydrates, such as starch and the various sugars and fats. The oxidation of these substances does not take place at ordinary temperature outside the plant, and the RESPIRATION 235 manner in which they are utilised within the tissues of plants during the respiration process is not understood. The oxidation is controlled and is dependent upon the protoplasm, for respira- tion ceases when life becomes extinct, and the amount and nature of the chemical changes carried on are not altered either by con- siderably reducing or increasing the amount of oxygen in the surrounding atmosphere. The absorption of oxygen and the subsequent emission of carbon dioxide are the beginning and end respectively of a long series of chemical changes, the intermediate stages of which are at present unknown. The disappearance of starch, sugars, fats and other carbon compounds during respiration is not due to simple direct oxida- tion probably the protoplasm itself is directly attacked by the ; absorbed oxygen after which it uses up the carbon compounds to repair its waste. The proportion of oxygen absorbed to the carbon dioxide gas given off is dependent on the energy of growth and on the materials consumed during respiration. In certain plants the -- -- ratio volume of carbon dioxide produ3ced voilume ofc oxygen consumed , has , been /� j found 1 to tb_ e as 1low as '3, while in others it has been observed as high as i"2. In germinating seeds, tubers and bulbs containing starch and sugars, and in most flowering plants, the volume of oxygen taken from the air during active normal respiration, is equal to that of the carbon dioxide exhaled ; but in the respiration carried on during the germination of seeds containing fats and oils, the volume of oxygen consumed is greater than that of the carbon dioxide exhaled, some of the oxygen absorbed by such seeds being apparently used up in oxidising the fats into some form of carbohydrate. It is by means of the energy set free by the oxidation of various compounds in the respiration-process that the plant is enabled to maintain its vital activity, and the vital energy of 236 RESPIRATION animals originates in a similar manner : when the physiological oxidation is prevented growth ceases, the streaming movement of the protoplasm within the cells is stopped, and the movements of the leaves, roots, stems and other organs of plants are suspended. In all cases heat is produced during respiration, and in warm- blooded animals it is easily perceived. In plants, oxidation is generally much less energetic than in animals, and the heat produced is so slight that no difference in temperature can be detected between green plants and that of the air surrounding them. Moreover, in ordinary green plants exposed to the air, the cooling effect of transpiration masks any slight rise in tem- perature due to respiration. However, when actively germinating seeds or rapidly expanding flowers and buds are heaped together, a rise of two or three degrees above that of the atmosphere may be readily observed, by placing the bulb of a thermometer among them. The amount of respiration is dependent on external and internal conditions, and in different parts of the same plant the activity of the process is not the same. In all young actively growing parts rich in protoplasm, such as germinating seeds, expanding buds and flowers, respiration is vigorously carried on, and the same is noticeable in injured cut portions of plants. In dormant bulbs, tubers and buds little or no respiration is observable. In dry seeds respiration seems to be entirely suspended, and many have been kept for twelve months in a vacuum, and in nitrogen and other gases under conditions which render respiration impossible, yet after such treatment the seeds germinated freely. At freezing-point and a degree or two below it, where growth is stopped, respiration may frequently be detected. With increasing temperature there is a steady increase in the amount of respiration up to the point where death takes place, and the process stops suddenly. RESPIRATION 237 Light appears to have no direct influence upon it, respiration continuing very similarly both in darkness and light. It has also been found by experiment that the process goes on quite normally even when the proportion of oxygen in the surrounding atmosphere is reduced to less than half that ordinarily present in the air. -- Ex. 136. Soak a handful or two of peas or barley grains in water for twelve hours. Take them out of the water and allow them to germinate on damp blotting-paper for twelve hours. Then put them in a wide-necked bottle, cork the latter and place it in a warm, dark room. Cork and place beside it another similar but empty bottle. Allow both to remain for twelve hours, after which time test for the presence of carbon dioxide by introducing a lighted match or taper into the bottles : the light is extinguished by carbon dioxide. Arrange another similar experiment, and test for carbon dioxide with lime-water : pour in the lime-water, and shake the bottles ; the lime- water becomes milky if carbon dioxide is present. -- Ex. 137. Partially fill a wide-necked bottle with half expanded young dandelion or daisy ' heads' ; cork and leave for twelve hours, after which time test for carbon dioxide as above. -- Ex. 138. Repeat the experiment above, using green leafy shoots, expand- ing buds, bulbs, tubers and other portions of plants. -- Ex. 139. Soak some peas for twelve hours, and after taking them out of the water allow them to germinate on damp blotting-paper for a few hours. Then place them in a flask ar- ranged on a retort stand, with a tightly fitting rubber stopper and bent glass tube as in Fig. 89. Slightly warm the flask with the hands and dip the open end of the tube (a) into mercury in a beaker (B). Leave the apparatus for ten or twenty minutes and fasten a piece of gummed paper on the tube (a) at a point {x) up to. which the mercury rises in it. Keep the whole in a room of even temperature for ten or twelve hours, and observe the position -of the mercury at the end of that time. If ^^'^- ^5- the volume of oxygen absorbed is equal to that of the carbon dioxide emitted, the mercury will remain at the same place in the tube. Repeat the experiment with oily seeds, such as hemp, linseed and turnip. 238 RESPIRATION RESPIRATION 239 however, bean seedlings and other plants emit the same or a greater volume of carbon dioxide when placed in an atmosphere free from oxygen, as they do when growing normally in the air. During intramolecular respiration carbohydrates and fats disappear from the tissues of the plants just as in ordinary respiration in the presence of abundance of oxygen, but the production of carbon dioxide is accompanied by the formation of alcohol and other compounds. The alcohol produced during the intramolecular respiration of ripe cherries amounted in one of Brefeld's experiments, to more than two per cent., and in pea seedlings to over five per cent of their fresh weight. While the higher plants are unable to maintain their vitality in the absence of free oxygen for more than a short time, many of the lower forms of plant hfe, such as yeasts and bacteria, are independent of the presence of free oxygen and continue to live and multiply without it (p. 772). CHAPTER XX. GROWTH. --We I. Growth. have seen in a previous chapter that at the apex of a stem or root of an ordinary green plant, there is usually z. formative region where the component small cells are in a state of division, and new cells are being manufactured. Immediately behind this is a longer or shorter portion which may be designated the growing region of the stem or root. Here the cells are found to be turgid, and in consequence of the pressure within them have increased in size, and at the same time many of them have become changed in form. These changes of size and form, owing to increased turgidity do not, however, necessarily constitute growth, although they are always associated with growth. Cells which are growing not only become distended by the osmotic pressure within the vacuoles, but also undergo a permanent change in size, form and structure, in consequence of the deposition of substances in their cell-walls and other parts; on withdrawing water from such cells the original state in which they existed when first produced in the formative region is not again reproduced by such a proceeding. Moreover, since the growth of a cell cannot go on without increased turgidity, and as this involves an addition of water to the vacuole of the cell, there is always an increase in the total weight of the cell when growth is proceed- ing : however, on account of the loss of substance by respiration, there may be a decrease in its dry weight if such loss is not compensated by anabolic nutritive processes. What is true of a single growing cell is also true in the case : GROWTH 241 of the whole growing region of a shoot or root, for the latter is merely composed of a number of active cells. Although it is not possible to define in a single sentence the exact meaning or connotation of the terra growth, it may generally be taken to imply a permanent change in the form of a living organism or some of its members, and that the region which is growing is also increasing in weight. The actual growing regions of the shoots developed in the dark from a potato tuber not only change their form but also, while they are growing, increase in weight at the expense of the water and reserve-food drawn from the tuber. It will be found, however, that the total weight of the tuber (which does not grow) and its growing shoots decreases in consequence of the loss of water by transpiration and by loss of carbon dioxide in the respiration process. During the early stages of its life when a plant emerges from the seed, growth takes place in all parts of its body. After a time, however, growth is confined to certain special localised portions, or growingpoints, and to the cylindrical cambiumtissue which brings about secondary growth in thickness of dicotyledonous stems. The growing-points in the case of stems and roots are generally terminal, or situated near the ends of these members : in such cases the youngest part is nearest, and the oldest part farthest away from the apex of the shoot or root. In the stems of grasses their increase in length is due to the activity of growing-points which are situated at the base of the internodes ; moreover the growth in length of the long leaves of onions and rushes, and that of many peduncles of flowers goes on at the base of the structures, their tips being the oldest parts growing-points of this character are described as intercalary. When a cell or a plant member begins to grow its rate of growth is at first slow; afterwards it grows more and more rapidly until a maximum rate is attained, after which the growth Q 242 GROWTH diminishes gradually until it ceases altogether when the part is mature. The time occupied by this gradual rise and fall is termed the grand period of growth. It is also noticed that the vigour or energy of growth of a stem or other member varies during the grand period : at one stage of the development of the complete stem the growing part either grows more rapidly or continues its growth longer than at another stage. For example, during the youngest stages of the development of most stems the energy of growth is low and short internodes are produced, later the energy increases, and larger internodes arise, afterwards the length of the internodes diminishes in consequence of a gradually decreasing energy of growth. -- Ex. 141. In autumn before the leaves have fallen, cut off branches from the common trees and shrubs, and measure the length between the several internodes on that part of each branch which has grown during the same season. Note the general rise and fall in the length of the internodes. Note also the relative size of the leaves at each node. Make similar measurements on the stems of annual herbaceous plants. Ex. 142. ^Repeat experiments 15 and 20: similarly mark with Indian A ink at intervals of inch the second and third leaves of a young onion plant soon after they appear; measure the intervals after the leaves have con- siderably lengthened, and compare the growth with that of a bean root. Is the region of greatest growth near the end of the leaf? -- Ex. 143. Select a stem of wheat or barley in which the ear is just ap- pearing ; cut about half-an-inch below the first and also below the second visible node from the top, so as to obtain about one internode of the stem. Remove the leaf-blade and a small portion of the leaf-sheath and carefully measure the total length of the stem and the small part of it below the node. Make five or six marks with Indian ink \ of an inch apart at the upper part of the stem. Then place the lower end of the stem in water, cover the whole if possible with a glass globe and leave it in a warm room for twenty-four hours ; or place the stem in a glass cylinder with a little water at the bottom for a similar period. Measure again the total length ; how much has the stem grown, and has the growth taken place near its upper marked end or near the base ? Has the small portion below the node grown at all ? Ex. 144.--Measure the length of the internodes on a few shoots of any ; CONDITIONS WHICH INFLUENCE GROWTH 243 vigorous common trees, shrubs or herbaceous plants in early summer when they are beginning to grow, and at intervals of two or three days for some time afterwards. Determine the time during which an internode continues to grow in length. -- 2. Conditions whicli influence growth. Only living plants grow and the cells of the growing parts must be in a youthful state. Various external conditions are also necessary for healthy -- growth, the chief of which are : (i) a suitable temperature (ii) an adequate supply of water (iii) appropriate food or food; materials (iv) the presence of oxygen, (v) Light although not ; absolutely essential to growth has a beneficial influence upon it. -- (i) Heat. It is well known that growth in winter, when the temperature of the surrounding aii and soil is low, goes on very slowly or not at all. As the temperature rises in spring seeds readily germinate and the buds of plants commence to grow ; with the increasing warmth of summer growth becomes more and more energetic. By subjecting a plant to a gradually decreasing temperature, a point is at last reached at which growth entirely ceases ; this is described as the minimum temperature for growth. It is not the same for all plants ; the seeds of many common weeds, and mustard and cress germinate, and the fully developed plants continue to grow near freezing-point, while those of the cereals are stopped when the temperature falls to about 5� C. On the other hand the seeds and plants of rnaize and the scarlet-runner bean cease to grow at about 10� C, while the minimum temperature for the germination and growth of the cucumber, melon, and many tropical plants is as high as 19� or 20� C. By raising the temperature from the minimum, a point is reached at which growth goes on most rapidly ; this is termed the optimum temperature. By further increasing the temperature beyond the latter point, growth becomes slower and slower until a maximum is attained, at which growth is entirely checked. 244 GROWTH Thus it is seen that plants may be too hot or too cold for growth, and between these extremes there is an optimum or best temperature where they make the most satisfactory progress. The optimum temperature for most common farm and garden plants is about 28� C, while the maximum usually lies between 38� and 43� C. ; the optimum for maize, scarlet-runner, bean and cucumber is about 33� or 34� C., the maximum about 46� C. It may be conveniently noticed here that although ordinary plants in an active state of growth have their development stopped at the temperatures indicated above, the death of the protoplasm does not usually take place until the higher temperature of about 56� C. is attained or until it has been cooled to freezing-point or several degrees below the latter. The power of withstanding heat and cold depends very largely upon the amount of water which the plant contains. Well-ripened shoots and buds containing little water do not suffer so much from the effects of frost during winter as sappy immature shoots which contain a larger proportion of water. Turgid seedlings, buds just opening and recently unfolded leaves, plants watered in the evening, succulent roots and all parts containing considerable amounts of water are often injured by exposure to sharp frost for a few nights. Usually when a plant is subjected to a temperature of 2� to 5� C. the cytoplasm allows a certain amount of pure water in the vacuole to ooze out of the cell into the surrounding