Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/65

Rh PHYSIOLOGY 111 ;rials VEGETABLE.] sugar (cane-sugar in the Beet root, glucose in the Onion, inulin in the Dahlia root, mannite in the unripe fruits of the Olive and in some Agarics, trehalose in many Agarics) are probably formed more or less directly from the glucose conveyed from other parts to the depository in each case. The fats occur as reserve materials characteristically in seeds and sometimes in fruits ; they are not stored up in any considerable quantity in any other kind of depository. They too are formed by the decomposition of protoplasm which has been constructed from plastic materials, nitrogenous and non-nitrogenous, which have been conveyed to the cells. M - The nitrogenous reserve materials are stored either in gens solution or as solid granules. In the former case they r s * e are amides, such as asparagin and glutamin, leucin and tyrosin, and are held in solution in the cell-sap ; they are present characteristically in roots and tubers, but they have also been found, though in small quantity, in seeds. In the latter case these materials are stored in the form of proteids, chiefly globulins and peptones, and the granules in which they are deposited are termed &quot; aleurone grains.&quot; The aleurone grain may consist simply of an amorphous mass of proteid, or a portion of the proteid may have crystallized out so as to form a crystalloid ; in most cases the grain contains a small mass of mineral matter which consists, according to Pfeffer, of double phosphate of lime and magnesia. Aleurone grains occur characteristically in seeds, and they are especially well developed in oily seeds. When once deposited, the reserve materials suffer no V ie of further change, or at most the proteids may slowly undergo ! e ve some alteration (globulin being converted into albuminate), so long as the organ in which they are deposited remains in an inactive condition. But when the external conditions become favourable the quiescent organ resumes its active life in a word, it germinates and the reserve materials which it contains then undergo chemical changes of such a nature as to convert them into substances which can readily travel to the seat of growth and can be used as plastic material by the growing cells. In a germinating seed, for instance, as the embryo grows the reserve materials of the seed diminish in quantity ; they are evidently conveyed to the seedling, and are used by it in the construction of new protoplasm. Beginning with the non-nitrogenous reserve materials, the starch in starchy seeds and the cellulose in such seeds as the Date are converted into sugar; this is proved by the detection of sugar as well in the seed as in the seedling, and by the detection in the seed of an unorganized ferment which possesses the property of con verting starch into sugar. In oily seeds the fats are replaced by starch, formed through the intermediation of protoplasm, and the starch so formed is converted into sugar. With regard to the reserve proteids, they are converted into amides, for it has been ascertained that, as they diminish in quantity, the amides, and notably asparagin, increase. The effect of the absorption of these plastic substances by the embryo is that the cell -sap of its cells becomes charged with them, for the supply is more rapid than the consumption in the formation of protoplasm. If the seed ling is growing under favourable conditions these sub stances gradually diminish in quantity. Some light has been thrown upon the nature of these conditions by the researches of Pfeffer. He found that Lupin seedlings grown in the dark contained a very large quantity of asparagin so long as they continued to live, but that if they were exposed to light the asparagin gradually diminished. But he ascertained further that mere exposure to light is not the cause of this, since the asparagin did not diminish in seedlings exposed to light in an atmosphere which con tained no carbon dioxide. The disappearance of the asparagin depended, therefore, upon conditions which were 55 essential to the formation of non-nitrogenous organic sub stance by the seedlings. Now the Lupin seed is one which is particularly rich in nitrogenous reserve materials, the quantity of non-nitrogenous reserve materials being relatively small. The accumulation of the asparagin in the seedlings grown in the dark is then to be ascribed to the absence of an adequate supply of non- nitrogenous substance with which it could combine to form proteid. When the seedlings were exposed to light this supply was forthcoming, and then the asparagin disappeared. Supply of Energy. It is evident that the various chemical processes which make up Supply the metabolism of plants involve an expenditure of energy ; hence of the maintenance of the life of the plant is dependent upon a supply energy, of energy. In the case of animals the food affords the principal supply of energy. It consists for the most part of complex organic substances which represent a considerable amount of potential energy, and when these substances are decomposed in the body the potential energy appears in the kinetic form. This holds good also with reference to plants which are destitute of chlorophyll, for their food necessarily includes, like that of animals, complex organic substances. But with plants which possess chlorophyll the case is entirely different. Their food consists of inorganic substances which do not represent any considerable amount of potential energy ; from these simple substances green plants build up complex organic substances which do represent a considerable amount of potential energy ; it is evident, therefore, that green plants must be largely supplied from without with kinetic energy in some form or other. It has been already mentioned that the metabolic processes of plants are materially affected by external conditions, especially by the pre sence or absence of light, and by variations in the temperature of the surrounding medium. A somewhat elevated temperature is essential to the active life of all plants, but light is essential only to the life of those which contain chlorophyll. This naturally suggests that the energy requisite for the maintenance of the life of plants is obtained by them in the form either of light or of heat. Light. In discussing the constructive metabolism of green plants Light a it was pointed out that such can only assimilate their food that source of is, can only construct protoplasm from it when exposed to light, plant- whereas plants which do not possess chlorophyll can assimilate energy, their food in the absence of light. It is true that a green seedling can live for a time in continuous darkness and increase in light, but it does so, not by assimilating its food, but at the expense of the organic reserve materials which may be present in it. The fact of the dependence of green plants upon exposure to light sug gests that the energy necessary for the processes of their constructive metabolism is obtained in the form of light, and that their chloro phyll enables them to avail themselves of this form of kinetic energy. The function of chlorophyll has been made clear by the researches of Timiriaseff and of Engelmann. They have shown that the evolution of oxygen by a plant containing chlorophyll, which is the expression of the first stages of constructive metabolism, is most active when the plant is exposed to those rays of the solar spectrum which correspond to the absorption-bands of the chloro phyll-spectrum ; the more conspicuous the absorption - band, the greater is the degree of activity, so that the evolution of oxygen is most considerable in the rays between the lines B and C of the solar spectrum, at the junction of the red and the orange, which correspond to the absorption-band I in the chlorophyll-spectrum. It is, then, in consequence of this absorption by the chlorophyll that the kinetic energy of the solar rays is made available for the work of constructive metabolism in the plant. The whole of the kinetic energy absorbed by the chlorophyll is not converted into potential energy ; still the chlorophyll-corpuscle appears to be a very perfect machine in this respect, for, according to Timiriaseff s calculations, it converts into the potential form as much as 40 per cent, of the absorbed energy. Inasmuch as light exercises so grsat an influence upon the constructive metabolism of green plants, it may be inferred that it must indirectly affect the absorption of food-materials by the roots. Rudolph Weber has, in fact, ascer tained that the greatest absorption of the essential ash-constituents takes place when the plant is kept exposed to those rays of light which are most efficacious in promoting its constructive metabolism. The effect of light upon the destructive metabolism of plants appears to be unimportant. This subject has been investigated by means of observations upon the respiration of plants ; and such a method is calculated to afford the necessary information, inasmuch as the activity of respiration may be taken as a measure of the activity of destructive metabolism. It has been generally stated that chlorophyll is not formed in the absence of light. There are, nevertheless, certain cases in which its formation in complete darkness has been observed, provided