56 PHYSIOLOGY [VEGETABLE. that the temperature has been sufficiently high, namely, in the cotyledons of some Conifers and in the leaves of Ferns. The colouring matter etiolin is formed in the corpuscles in darkness, but the conversion of this into chlorophyll can only take place, as a rule, under the influence of light. The formation of chlorophyll will take place in light of very low intensity, but, as Wiesner s experiments show, there is a lower limit of intensity be-low which light is inactive. With regard to the relative efficacy of the differ ent rays of the spectrum in promoting the formation of chlorophyll, it appears from Wiesner s researches that all the rays between Fraunhofer s lines B and H promote it in different degrees, and further, in confirmation of older observations, that seedlings turn green more rapidly in the yellow than in any other part of the spectrum. This last statement is true only for light of moderate intensity; when the light is very intense the formation of chlorophyll takes place more rapidly in blue than in yellow light. The reason of this is that in intense light chlorophyll undergoes decomposition, or at least chemical alteration of the nature of oxidation, which goes on most actively in yellow light. Heat a Heat. Plants behave in relation to temperature like the cold- source of blooded animals. When they are maintained at a low temperature energy, they cease to exhibit any signs of life. The meaning of this is that at a low temperature the activity of the metabolic processes is so reduced that they appear to be altogether arrested. But the im portance of a moderately high temperature for the maintenance of the active life of the plant is not, as might be supposed, that it affords a continuous supply of energy to be converted into work ; it is rather that it determines the initiation of chemical processes which are carried on by means of energy obtained from other sources. Hence the supply of energy in the form of heat is rela tively small as compared, on the one hand, with the supply of potential energy afforded by their food to the plants which do not possess chlorophyll, and, on the other hand, with the supply ob tained in the form of light by plants which do possess chlorophyll. It is not possible within the limits of this article to enter fully into the relations existing between plant -life and temperature. The following statements will at least indicate their general nature. In the first place, the tolerance of extreme temperatures is different for different plants, as determined in the case of any particular organ, such as the seeds for instance. Secondly, for each of the processes which can be studied separately, such as germination, growth, respiration, the formation of chlorophyll, the action of unorganized ferments, the evolution of oxygen by green plants in light, &c. , there are three cardinal points of temperature to be noted the minimum or zero point, at which the performance of the process is just possible ; the optimum point, at which it is carried on with the greatest activity ; the maximum point, at which it is arrested. But these different phenomena do not all stand in precisely the same relation to temperature, that is, the cardinal points for the exhibition of any two or more of these phenomena by one and the same plant do not necessarily coincide. Thirdly, the larger the proportion of water in an organ, the more liable it is to be injured by exposure to extreme temperatures. Expenditure of Energy. We have now to ascertain what becomes of energy supplied to the plant. The matter may be briefly stated thus : a portion of it is stored up in the plant in the form of potential energy ; the re mainder is lost to the plant, being either spent in the performance of mechanical work in connexion with growth or movement, or given off, most generally in the form of heat, occasionally in the form of light, and possibly in the form of electricity. The stomig- up of energy in the potential form may be termed the " accumula tion of energy," the loss as the "dissipation of energy." Accumu- 1. Accumulation of Energy. The accumulation of energy is the lation of necessary accompaniment of constructive metabolism ; the forma- energy. tion of more and more complex organic substances involves the conversion of kinetic into potential energy. By taking into con sideration the amount of organic substance formed by a plant from its first development to its death, it is possible to arrive at some idea of the amount of kinetic energy which the plant has stored up in the potential form. For the heat which is given out by burning the organic substance is but the conversion into kinetic energy of the potential energy stored up in the substance ; it is but the reappearance of the kinetic energy which was used in produc ing the substance. The heat, for instance, which is given out by burning wood or coal represents the kinetic energy, derived prin cipally from the sun s rays, by which were effected the processes of constructive metabolism of which the wood or the coal was the product. The amount of energy thus stored up by plants in the potential form is very large, because they produce relatively large quantities of organic substance. Dissipa- 2. Dissipation of Energy. The expenditure of energy in con- tion of nexion with growth and movement, and with the evolution of energy, heat, light, and electricity, is dependent upon destructive meta bolism, for the conditions which are essential to destructive meta bolism are also those which are essential to the exhibition of these phenomena. Taking growth, for example that is, continuous change of form accompanied usually by increase in bulk it appears that in an aerobiotic plant it is dependent upon the following ex ternal conditions, namely, a supply of free oxygen and an adequate temperature, conditions which are precisely those upon which the destructive metabolic processes of such a plant also depend. This is true in such plants of the other above-mentioned phenomena also. Anaerobiotic plants can grow when the conditions are such that they can induce active fermentation, that is, when their destruc tive metabolism is active. After what has been said in the section on the " Nervous System " above (p. 38 sq.) about animal movement it is hardly necessary to prove that the movements of plants, which are of essentially the same nature, as those of animals, depend upon destructive metabolism and involve a dissipation of energy. An evolution of energy in the form of heat is the inseparable result of destructive metabolism. With regard to plants, it may be stated generally that the evolution of heat is not sufficiently active to raise the temperature of the plant-body above that of the surrounding medium, it being remembered that plants are constantly losing heat, principally by radiation and in connexion with tran spiration. In organs, however, in which destructive metabolism is very active it is easy to detect a rise of temperature, especially when a large number of them are collected together. A good instance of this is afforded by germinating seeds ; for example, a rise of temperature is a familiar fact in the process of the malting of Barley. It can also be readily observed in the case of opening flowers in dense inflorescences ; Warming observed, for example, that, at the time of the opening of the flowers, the inflorescence of an Aroid (Philodendron bipinnatifidum} attained a temperature of 18 5 C. above that of the air. The evolution of light by plants is a phenomenon which has been known from the times of Aristotle and of Pliny, and is commonly spoken of as " phosphorescence. " All the well-authenticated in stances of luminosity are confined to the Fungi, to various Agarics, and to Schizomycetes (Bacteria). The so-called " phosphorescence " of decaying wood is due to the presence of the mycelium of Agaricus melleus (Rhizomorpha), and that of putrefying meat and vegetables to micrococci. See PHOSPHORESCENCE. The evolution of light is essentially dependent upon the life of the organism, and further, it is dependent upon the destructive metabolism ; for it ceases when the organism is killed (as by dipping it into hot water), or deprived of its supply of free oxygen, which is essential to the metabolic processes. In view of the changes, both chemical and physical, which are going on with greater or less activity in the various parts of a liv ing plant it has not been unnaturally inferred that the electrical equilibrium is being constantly disturbed, and that differences of electrical potential energy may exist in different parts. Many experimenters have investigated this subject, and such differences have been apparently observed. It is impossible to enter here into a detailed consideration of the results obtained ; it may suffice to state that in the majority of cases the electrical currents detected do not indicate a dissipation of the energy of the plant, but are due to physical causes, and in some cases even to the effect upon the organism of the apparatus employed for the purpose of detecting them. It has been clearly made out in certain instances that the currents persist in organs which have been suddenly killed in such a way as not to destroy their gross organization. There is, however, one instance in which an electrical current has been detected which seems to be connected with the destructive metabolism of the plant. Bunion-Sanderson and Munk have both observed that, when the two electrodes are placed upon a mobile leaf of Dioniea muscipula (Venus s Fly-trap) when at rest, a certain electrical current is indicated by the galvanometer. When the leaf is stimulated, whether the stimulation be or be not followed by a movement, the direction of the observed current is suddenly re versed. This change in the direction of the current or " negative variation," as it is termed is, according to Burdon-Sanderson, the " visible sign of an unknown molecular process, which he considers to be "an explosive molecular change," of the same nature as the negative variation which follows upon the stimulation of the muscles and nerves of animals. In concluding this part of the subject it may be well, for the Incom* sake of clearness, to draw up an account of the income and expendi- and ex ture of a plant. pendi- In the case of a plant possessing chlorophyll the income of matter ture ol consists of the food (salts, water, carbon dioxide, free oxygen), ami plants. the income of energy of kinetic energy in the form of light and heat, the former being the more important of the two items. The great bulk of the food absorbed is converted into organic matter, and is for the most part retained by the plant in the form of organ ized structures, of reserve materials, and of waste-products which are not excreted ; but a certain proportion of it is lost in the form of the carbon dioxide and water exhaled in respiration, of oxygen exhaled by green parts in sunlight, and of excreted organic or inorganic matter. Besides these items of loss there are yet others.
All plants lose a certain amount of matter in connexion with repro-