312 



LECTURE XIX. 



by a detailed investigation in my laboratory, that on increasing the carbon dioxide of 

 the air to 5—10% leaves (of Glyceria spectabilis, Typha latifolia, and Oleander) 

 evolve the maximum amount of oxygen in intense light. 



On the basis of all these facts, there cannot be the slightest doubt that the 

 starch in the chlorophyll-corpuscles is to be regarded as the first evident product of 

 assimilation; and that it continually undergoes solution in the light as well as 

 in darkness, and becomes distributed in some form from the assimilating organs 

 into the tissues of the plant. There it is either used up immediately for the growth 

 of new organs, or is stored up as reserve-materials in seeds, tubers, bulbs, root-stocks, 

 and the cortex and wood of trees ; or it provides the material for the formation of 

 other organic compounds, and especially for the synthesis of the proteid substances. 



Since, according to my theory, the basis for the whole of the organic substance 

 of a normal plant is provided by the formation of starch in the chlorophyll of 

 {he organs of assimilation, and, above all, the whole of the carbon, in whatever 

 organic combination it may be found later, occurs originally in the form of starch, 

 it is particularly interesting to enquire how great the work of the assimilating chlo- 

 rophyll may be under definite, and especially under normal conditions of illumination. 

 The question, however, can so far only be approximately answered, since we cannot 

 employ individual chlorophyll-corpuscles for such observations— nor even individual 

 cells, the unicellular Algse not being suitable for quantitative determinations. 

 Moreover, it is not impossible that the quantitative activity of chlorophyll-corpuscles 

 similar in size and colour may be peculiar for each species of plant ; and- 

 some observations make such an assumption almost probable. Since in the 

 course of about 100 summer days a huge plant, the dry- weight of which 

 may attain two or three kilograms, is developed from the tiny embryo of 

 a Tobacco seed, while from the much larger seed of a Fir, for instance, there 

 arises in the same time a small plantlet, the dry-weight of which only amounts 

 to a few grams, it may be supposed that, among the very numerous causes 

 which may eflfect this difference, the activity of the chlorophyll-substance itself is 

 possibly much more energetic in the first example than in the latter. This, how- 

 ever, is not the place to enter more deeply into so difficult a question. But it 

 is desirable to know, even if only approximately, how much starch may be 

 assimilated in the chlorophyll under favourable circumstances. Such consider- 

 ations impelled me in 1878 to persuade Dr. Weber, then my assistant, to 

 undertake an experimental investigation, in the first place to decide only the 

 question how great is the quantity of starch assimilated in a given time (e.g. 

 ten hours of daylight) in the summer and with favourable illumination, by a 

 certain amount of leaf-surface (e. g. a square metre ^). Even in this very simple 

 form the answer to the question presents great experimental difficulties. This is 

 particularly the case because the starch at first assimilated by the few leaves of the 

 experimental plants is at once used up again for the production of new organs 

 of assimilation ; the area of the leaves enlarges froih hour to hour, and from day 

 to day; and older leaves cease to assimilate, and younger ones begin to perform 



' Weber, ' Ueber sfectfische Assimilationsenergie' Arbeiten des bot. Inst, in Wurzburg, II, 

 p. 346. 



