310 LECTURE XIX. 



the renewed formation of starch by a second illumination. I made these experi- 

 ments with Begonia, Tobacco, and Geranium peltaiwn, placing the entire plants in 

 the dark until the starch had disappeared from the chlorophyll of the leaves; 

 then, the plants being again exposed to the light, a renewed formation of starch 

 resulted. In the first experiments with Begonia, certain portions of the leaves were 

 covered with black paper on both sides, with the result that the starch disappeared 

 from the chlorophyll of the leaves only at these places. I now employ this form 

 of experiment in my lectures on vegetable physiology, in order to demonstrate 

 the influence of light on the formation of starch ; or, better, of darkness on the 

 disappearance of starch. It suffices for instance to fasten a broad band of tin- 

 foil or lead in summer on plants with conveniently large leaves and growing in pots 

 — e.g. Tobacco, Maize, Canna, etc., without depriving the plants of light. After 

 a few days, the leaves so treated are cut off", and thrown for a few minutes into 

 boiling water in order to kill them, and to cause the starch in the chlorophyll 

 to swell. They are then placed for some hours in strong alcohol, which removes 

 the chlorophyll colouring-matter, and the now colourless leaves are finally placed 

 in a vessel containing a weak, pale brown, alcoholic solution of iodine. After 

 a short time, the parts of the leaf which were not shaded from the light appear 

 blue-black, owing to the formation of iodide of starch : the place shaded by the 

 band of tinfoil, on the other hand, remains colourless, simply because the chlorophyll- 

 corpuscles there contain no more starch. 



This experiment demonstrates yet another fact, viz., that the action of light 

 is strictly local, and is not transferred to neighbouring shaded places. Moreover 

 the action of carbon dioxide is also strictly local, as Moll showed in 1878'. 

 The leaves produce starch only at the places directly in contact with air con- 

 taining carbon dioxide. If, for example, one half of a leaf is placed in a 

 space the air of which is deprived of carbon dioxide, while the other half 

 remains in the ordinary atmosphere, both halves being equally illuminated, starch 

 arises in the chlorophyll only in the last-named half, the other producing none. 

 Again, if the leaves of a plant rooted in garden soil are placed in a space 

 which contains no carbon dioxide, no starch is produced in the chlorophyll, 

 even with favourable illumination. These experiments prove that the view 

 entertained until quite recently by the older physiologists, that carbon dioxide 

 may be conveyed from the roots into the leaves, and there assimilated, is 

 absolutely wrong. 



In some cases it is impossible directly to observe starch as the product of 

 assimilation in the chlorophyll-grains. I found this to be the case in the 

 leaves of our common Onion {Alliimi Cepa), where, however, large quantities of 

 glucose (sugar) are to be recognised as the result of assimilation. As a rule 

 this plant does not form starch, and the reserve-material in its bulbs also con- 

 sists of glucose. It was observed later, that in the leaves of Sirelilzia and Musa 

 also fatty oils are found as a rule in the chlorophyll instead of starch. The 

 assumption that the first product of assimilation of the chlorophyll is in these 



' Moll, * Ueber die Herkunft 3es Kohlenstoffs in der Pßanze.' Arbeiten des bot. Inst, in Würz- 

 burg, II, 1878, p. 105. See also Vines, ibid., p. 121. 



