THE ASSIMILA TION OF CARBON B Y AUTOTROPHIC PLANTS. I in 



starch has been known by far the longest, because it is the most striking and 



obvious constituent in the chloroplast. With the aid of a microscope we can often 



demonstrate its presence in the chloroplasts after a very brief exposure to 



light, if we take the precaution of seeing that the plastids were free from starch 



at the beginning of the experiment. We further find that when the plant has 



been kept for some time in the dark, not only is a further formation of starch 



prevented, but dissolution of what was present takes place. When the leaf, 



freed from starch and still attached to the stem, is placed in the sun early on 



a bright summer morning, we can, by using iodine solution, demonstrate the 



hourly increrse in amount of starch. In the evening the chloroplasts are 



found to be so packed with starch grains that iodine solution turns the leaf 



perfectly black. In order to employ the iodine-proof to the best advantage it 



is advisable to extract the chlorophyll with hot alcohol, so that that pigment 



does not interfere with the colour reaction ; for the proper demonstration of 



small quantities of starch it is preferable to treat the preparation either with 



chloralhydrate or with boiling water. When leaves of different plants are 



subjected to similar treatment with iodine, the resulting coloration gives us 



an approximate measure of the amount of starch formed, and it is easily seen 



that not all plants are able to produce starch to the same extent. A. MEYER 



(1885) has published a list from which one can see not only that variations 



occur in the capacity for producing starch, but also that their variations are 



characteristic of certain families. MEYER found : 



1. Very large quantities of starch in the Solanaceae and Papilionaceae. 



2. Large quantities in the Papaveraceae, Crassulaceae, Geraniaceae, Oxa- 

 lidaceae, Boraginaceae, Labiatae, Dioscoreaceae, and many others. 



3. Moderate amounts in the Caryophyllaceae, Ranunculaceae, Coniferae, &c. 



4. Small amounts in many Lobeliaceae. 



5. Very little in many Gentianaceae and Iridaceae. 



6. None at all in Asdepias cornuti ; Allium, Scilla and many other 

 Liliaceae, and in many Amaryllidaceae and Orchidaceae. 



It is easily demonstrable, however, that even where no starch occurs there 

 is still an active evolution of oxygen in sunlight, and that here also the volume 

 of oxygen given off is equal to the amount of carbon-dioxide decomposed. It 

 follows at once that other carbohydrates are also produced. More exact in- 

 vestigation further shows us that, even in the plants which are richest in starch, 

 the starch is not the first product of assimilation. In the first place, it is highly 

 improbable that an insoluble complex body like starch should arise at once, 

 and in the second place, one finds even in favourable instances that the starch 

 appears only at an appreciable interval after the commencement of the de- 

 composition of carbon-dioxide. Thus KRAUS (1869) found the first visible 

 traces of starch in Spirogyra five minutes after the illumination of the cells, ai 

 in other cases after a longer interval ; but, by means of the bacterium met! 

 it may be shown that the decomposition of the carbon-dioxide takes pla 

 simultaneously with illumination. The first products of assimilation must 

 manifestly be soluble, and from these starch arises as a secondary result, 

 proof of this fact we owe to A. MEYER (1885), who has shown, by chemical 

 analysis, that during assimilation large quantites of soluble carbohydrate 

 formed in plants without starch appearing, some reducing, some non-n 

 and further, that the same substances occur in plants which possess abundant 

 starch. These results have been confirmed by A. F W. SCHIMPER (1885), wh 

 draws the following conclusion from them, viz. that the d**^***** 

 plants in the matter of starch formation does not consist in a difference 

 activity of the assimilative process, but in the fact that while one sen 

 stores the soluble carbohydrates as such the other transforms therr 

 starch. [According to A. MULLER (Jahrb. f. wiss. Bot. 1904, 40, 443), leaves con- 



