Host and Parasite in certain Diseases of Plants. 



405 



oxygen) form new combinations, which result in the production of 

 carbohydrates, which then separate from the protoplasm.* 



We are here, of course less concerned with the difficulties which 

 beset the questions, what rays of light are concerned in this process,, 

 how their energy is employed in the chlorophyll, and what part the 

 chlorophyll itself takes directly in the process ; or with questions as 

 to the exact products formed during the putting together of the 

 carbohydrate in the protoplasm of the chlorophyll corpuscle, and so 

 on, than with certain well-established facts and conclusions, such as- 

 the following. 



The process of building up the products obtained by the decompo- 

 sition of the carbon dioxide and water in the protoplasm into carbo- 

 hydrates goes on continuously in the sunlight, so long as it is 

 sufficiently intense, and the excess beyond what is immediately re- 

 quired for the nourishment and respiration (i.e., the maintenance of 

 metabolic activity) of the living substances of the cell takes the final 

 form (usuallyf) of starch. Free oxygen escapes all the time, and, in 

 so far as this is not absorbed for purposes of oxidation, there and then 

 in the cell, this oxygen goes to enrich the atmosphere. Moreover, 

 these temporary stores of starch are continuously being transformed 

 into soluble glucoses, by means of diastatic ferments]; in the proto- 

 plasm ; this process goes cn day and night, and its result may be 

 easily demonstrated in the case of leaves removed from the plant after 

 exposure to the sunlight during the day. After a few hours in a 

 warm, dark, normal atmosphere, relatively large quantities of glucose- 

 are found in the cells, while the starch is disappearing. This glucose, 

 I need hardly remind you, is the soluble movable form of the carbo- 

 hydrates^ and it is worked up again, so far as it is in excess of the 



* The literature of this part of the subject is enormous, and dates from Priestley 

 (' Phil. Trans.,' 1772) to the present time. It may be said to fall under four heads : 

 (1) the nature and functions of chlorophyll ; (2) the absorption of carbon dioxide 

 and the evolution of oxygen ; (3) the intensity and kind of light necessary ; (4) the 

 chemical processes which intervene between the coming together of the carbon 

 dioxide and water and the production of the final visible product — starch. I shall, 

 naturally, here refer only to such special literature as bears on the main subject of 

 the present lecture. 



f Sachs, 'Flora.,' 1862, Nos. 11 and 21, and 1863, p. 33 ; also £ Bot. Zeitg.,' 1862, 

 col. 366 ; Godlewski, ' Flora,' 1873, p. 378, and ' Arb. des Bot. Inst, in Wurzburg,' 

 1873, vol. 1, p. 343. Again, Sachs, ' Arb. des Bot. Inst. Wurzburg,' vol. 3, Heft 1, 

 1884 ; G-. Kraus, 1 Jahrb. fur wiss. Bot.,' vol. 7, 1870, p. 511 ; Famintzin, ' Jahrb. fur 

 wiss. Bot.,' vol. 6, p. 34. 



X Baranetzky, ' Die Starkeumbildenden Fermente,' 1878. 



§ Numerous interesting results have been obtained of late years confirming and 

 strengthening our theory of carbohydrate assimilation : see Bohm (' Bot. Zeitg.,' 1883, 

 col. 33), A. Meyer (' Bot. Zeitg.,' 1886, col. 81), Laurent (' Bot. Zeitg.,' 1886, col. 151), 

 who proved that leaves deprived of starch can form it from various sugars, glycerine,. 

 &c. ; also Wehmer ( £ Bot. Zeitg.,' 1887, col. 713), O. Low (' Ber. d. Deutsch. Cherm 



