Sachs (1862) was the first to relate the carbohydrates appearing in the leaf to the process of assimilation; he was responsible for the dictum that starch was the "first visible product of assimilation." This was based on the fact that in the case of many plants starch appeared in the plastids of the leaf in the light and disappeared again in the dark. It is now generally agreed that carbohydrates of some kind are the main products of photosynthesis, though many monocotyledons form little or no starch, but much sugar in the form of glucose, fructose and cane sugar. Which carbohydrate is first formed is a matter in dispute. In green leaves exposed to light cane sugar accumulates markedly; this has led to the view that it is the first formed sugar. A much more plausible hypothesis would seem to be that hexose sugars (glucose and fructose) are first produced, and that when these reach a certain concentration cane sugar is formed from them ; this view is supported by the work of Weevers (1924). The starch arises in all probability from glucose and it would seem likely that there is a critical concentration of sugar in the chloroplast above which starch is formed from the sugar, and below which the starch is converted back into sugar. This is confirmed by the fact that by floating leaves on sugar solutions of high concentration even those which are normally starch-free will produce that substance in their chloroplasts.
Effect of Various Conditions on the Rate of Photosynthe sis.—The rate of this process is affected by the concentration of carbon dioxide available, by the amount of water, by the intensity of the light and by its wave length, by the chlorophyll content, by the supply of oxygen and of mineral salts, and also by many other internal conditions, or factors as they are called. It used to be supposed that all these factors acted independently, but it was shown by the work of F. F. Blackman (1905), that the factors are closely interrelated. He put forward the "theory of limiting factors," that in any set of conditions the rate of a physiological process was determined by the factor present at the lowest inten sity. The view that under any set of conditions it was only by the increase of the intensity of one factor that the rate of the process could be increased requires some modification in the light of recent work. It is clear, however, that if any factor A is present at very low intensity it will prevent an increase of another factor B having the marked effect upon the rate of the process which it would have produced if the factor A were acting at a higher intensity. Bearing this relationship of the factors in mind, we find that with increasing concentration of carbon dioxide the rate of assimilation goes up proportionally, if the concentration is not too high and light and temperature are sufficient. Similarly with increasing light-intensity the rate of assimilation goes up proportionally if the temperature and concentration of carbon dioxide are sufficient. With high light intensities there is prob ably a falling off in the rate, as with higher concentrations of car bon dioxide. With a moderate temperature, if other conditions are favourable, the rate of assimilation obeys the Van't Hoff rule, approximately doubling for each rise of temperature of io° C. At higher temperatures, somewhere in the neighbourhood of 25° C, with some plants, the injurious effect of high temperature appears and the rate begins to fall off. The process of assimilation is little
sensitive to the concentration of oxygen, reduction to
of the normal amount having no effect, but complete absence of oxygen acts injuriously. A number of internal factors must markedly affect the rate of photosynthesis, but the only one that has been closely investigated is that of the chlorophyll-content, by Willstater and Stoll. It was found that the photosynthetic rate per unit of chlorophyll (the "assimilation number") varied markedly in different plants and even in the same plant. Leaves of yellow varieties were found to show very high rates in propor tion to the amount of chlorophyll present, though the rate in these leaves is less influenced by temperature than is that of fully green leaves. Clearly there is some internal factor (or factors) other than chlorophyll content which is controlling the rate. This may be termed the "protoplasmic factor"; Willstatter and Stoll con clude that this factor is enzymic.
question of the effectiveness in photosynthesis of light of different wave lengths has been hotly debated by physiologists for many years. The question is a complicated one since in white light there are great differences in the energy-value of the rays of different wave lengths. and the different rays are absorbed to a very different degree by the chlorophyll and the colourless parts of the leaf. In the case of the leaf there is little doubt that with equal intensity of incident light the red rays are more effective than the blue. The work of Warburg and Negelein quoted below indicates also that for equal quantities of light-energy absorbed by the chloroplast, the red rays are considerably more effective than the blue.
is known that the green leaf is comparatively inefficient as a producer of assimi lation material. Brown and Escombe (1905) attempted to deter mine the efficiency of the green leaf by comparing the energy of the light absorbed with the absorption of carbon dioxide, the assumption being made that glucose was produced in the leaf from the carbon dioxide. Their results are only very approximate but they indicate that the efficiency is only about 15-, increasing however with low intensities of light to about A large
portion of the energy absorbed is used in transpiration. and with the higher intensities of light the concentration of carbon dioxide in nature is too low to allow of the light being fully effective. The most accurate experiments on the efficiency of the photosynthetic process are those of Warburg- and Negelein (1922 and 1923) with the minute unicellular fresh water alga, Chlorella. With such a plant there is no question of transpiration. and the experimental arrangements were such that practically the whole of the light received was absorbed by the chloroplasts of the cells; in the case of the leaf a considerable amount of the light passes through it and a certain amount is reflected. Warburg and Negelein show that with decrease in the intensity of the light the efficiency goes up, i.e., a larger proportion of the light absorbed is made use of in
the rate of the latter process being measured by the kroduction of oxygen. In their earlier experiments they ob altogether satisfactory. In their later work
obtained results Whether of 59% for red light, and 34% for blue light. hether these
centages are the maxima that can be obtained is not yet certain.