VEGETABLE.] PHYSIOLOGY 57 duction, for all plants throw off in the course of their lives certain portions of their structure in the form of seeds, spores, antherozoids, &c. Again, plants which persist for more than one period of growth lose matter by the falling off of certain of their organs and of por tions of their structure, for example, by the falling of the leaves in autumn, and by the shedding of bark, fruits, &c. With reference to the expenditure of energy, a large proportion of the income of energy remains stored up in the potential form in the organic matter which the plant accumulates. A dissipation of energy as heat and in connexion with growth is common to all plants : in some there is dissipation of energy in the form of motion, in some in the form of light, in some, probably, in the form of electricity. A loss of energy potential energy occurs also when the plant loses organic matter in any of the ways mentioned above. These various items may be tabulated under the two heads of " income " and " expendi ture. " The water lost in transpiration is not considered, for it simply traverses the plant ; only that amount of water is considered which may be assumed to enter into the processes of constructive metabolism or to be produced in the processes of destructive metabolism. Plant possessing Chlorophyll. Income. Matter. Food- Inorganic salts. Carbon dioxide. Water. Free oxygen. Energy. Kays of light absorbed by chloro- phyll. Heat. Expenditure. Matter. ^ Organic substance formed. Carbon dioxide ) evolved in respir- Water I ation. Free oxygen, evolved in light. Excreted substances, organic or inorganic. Reproduction (spores, seeds, &c.). Other losses (Iwives, fruits, bark, &c.). Energy. Constructive metabolism. Growth. Movement(in some cases). Heat. Light. Electricity (in some cases). Potential energy (when organic matter is excreted or thrown off). Balance in favour of Plant. Matter. Organic substance (including tissues, reserve materials, and uuex- creted waste-products). Energy. Potential energy, represented by the accumulated organic sub stance. Plant destitute of Chlorophyll. Income. Expenditure. ame as above, except that no free oxygen is given off. Matter. Food- Inorganic salts. Organic substances. Water. Free oxygen (in most cases). Energy. Potential energy of organic food. Heat. Balance in favour of Plant. Same items as above. Movement of Plants. !ve- It has been pointed out above that movement, including in the int. conception the slow movement of growth, is an item in the ex penditure of energy by the plant. The phenomena connected with movement are of such physiological importance that it will be well to consider them rather fully. In dealing with this large subject attention will be directed for the present simply to the external phenomena, leaving the internal causes and mechanisms till subsequently, and those presented by growing organs will be taken first. 1. Growth. In commencing the study of growth it is important to have a perfectly clear idea of what the word means. It means the continual change in form of the body of the plant, or of any organ of it, the change being frequently accompanied by increase in bulk, though this is not necessarily the case. For the purposes of this article it will be convenient to use "growth " as meaning, unless expressly stated otherwise, growth in length, that is, the elongation of the organ along the line joining its base and its apex. The conditions upon which growth is dependent are (1) a supply of plastic material for the formation of new protoplasm ; (2) favour able external conditions, especially an adequate temperature ; (3) a supply of free oxygen in the case of aerobiotic plants, or, in the case of anaerobiotic plants, of fermentable substance ; (4) a supply of water to maintain the turgidity of the cells. Any variation in these essential conditions will lead to a variation in the rate of growth. The capacity for growth is limited, as a rule, to a certain period of the life of an organ and of its constituent cells ; when this period is past growth ceases, however favourable the external con ditions may continue to be. ^ate of The rate of growth of an organ is not uniform. At first the organ rowth. grows slowly, then more and more rapidly, until a maximum rapidity is reached, and then the rate diminishes until growth ceases altogether. This cycle of spontaneous variation in the rate of growth is known as the "grand period of growth." It can be conveniently studied by marking on the growing point of an organ a series of transverse zones of known length, and observing their relative elongation in a given time. It will be found that the youngest (nearest the apex) have elongated slightly, that the elon gation is greater the farther each successive zone is from the apex, until a zone of maximum elongation is reached ; the elongation of the zones lying behind this will be found to be less and less, until at last zones will be found which have not elongated at all. In addition to the variations in the rate of growth in length of an organ which make up its grand period it is found, if its growth be watched from hour to hour, or at even shorter intervals, that it presents irregular variations, which are likewise to be regarded as spontaneous. Variations in the rate of growth may be induced by variations in the external conditions, especially by variations of temperature and of illumination. It will be of interest to inquire briefly into these relations between growth and temperature and growth and light. Inasmuch as the decompositions which determine the evolution Growtli of energy in the plant are dependent upon temperature, their and tern- activity being promoted by a rise of temperature within certain perature. limits, it will be readily understood that growth, which is one ex pression of the evolution of energy, should likewise be affected by variations in temperature. It has been found, in fact, that the growth of any given plant will only take place within certain limits of temperature, a lowest or minimum temperature on the one hand and a highest or maximum temperature on the other ; and further, that between these two points there is one, the optimum temperature, at which the rate of growth is most rapid. Growth is more rapid at each degree as the temperature rises from the minimum to the optimum point ; it is less rapid at each degree as the temperature continues to rise from the optimum to the maxi mum point ; and conversely, growth is more rapid at each degree as the temperature falls from tlie maximum to the optimum, and less rapid at each degree as the temperature further falls from the optimum to the minimum. This dependence of growth on temper ature, and this relation between different degrees of temperature and different rates of growth, may be conveniently spoken of as the "tonic influence" of temperature. The mere variation in temperature as such does not appear, as a rule, to affect the rate of growth. Roots exposed to rapid and considerable variations of temperature for some time are found by Pedcrsen to have grown to about the same extent as similar roots which had been growing for the same time at the mean temperature. The only case in which it appears that variation in temperature produces a distinct effect is afforded by Pfeffer s observations upon the opening and closing of flowers. He found that a rise of temperature caused the flowers to open and a fall to close, the opening or closing being an expression of the accelerated growth in length of the organ as a whole. This effect of variation of temperature is distinguished as the "stimulating" effect. In considering the relation of light to growth we have princi- Growth pally to consider its influence as being an essential normal condition and of growth, its " tonic " influence, that is, it is a question whether or light, not light exercises any influence which can be regarded as "stimu lating" on the rate of growth. Speaking generally, it may be stated that plant-organs, with the exception of ordinary flattened horizontally expanded leaves and other organs of similar organiza tion, grow at least as well in darkness as in light, that exposure to light is not an essential condition of their growth. With leaves and leaf-like organs the case is different. When plants are kept for some time in darkness one of the most striking features is the smallness of the leaves of the shoots which have been developed during that time. This is not to be ascribed to an absence of plastic material, for it is exhibited when plastic mnterial is abun dantly present in the tissues ; nor can it be attributed to the fact that in darkness the leaves are not able to carry on the formation of organic substance, for it is not all leaves which remain small in darkness, but only those which have the organization described above. The long tubular leaves of the Onion, for example, con tinue to grow in darkness, and so do the long flattened leaves of Irises. The arrest of the growth of flattened horizontally ex panded leaves in darkness is due to some peculiar effect, which we must regard as of a "tonic" nature, exercised by light upon the growing cells. Intermittent exposure to light for brief periods suffices to enable the leaves to carry on their growth in darkness, and it is not necessary that the light should be intense. The in termittent exposure induces in the leaf a condition, though it can not be precisely stated what, which permits of the continuance of growth, a condition which is termed "phototonus." Assuming that the organ is actually growing, we find that in all cases light retards the rate of growth, and this the more markedly the greater its intensity. Wiesner has, in fact, shown that growth may be altogether arrested by exposure of the growing organ to intense light. The effect of light in retarding growth has been ascertained by comparative measurements of similar organs growing, some in darkness others in light, and is proved negatively by the greater length usually attained by shoots which have grown in darkness for a given time as compared with that attained in an equal time by shoots growing in light. It appears that variations in the in tensity of light, as such, affect the rate of growth. Pfeffer has
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