THE FOOD OF PLANTS 



The above conclusions are all supported by direct observations and 

 experiments. Thus starch appears as an assimilatory product in most but 

 not in all plants, either soon after assimilation commences, or not until 

 a larger or smaller amount of sugar has accumulated, while in some cases 

 no starch is produced, however much sugar may be present (Sect. 55). 

 Instead of starch a certain amount of oil may be formed in some 

 plants, and in many leaves a marked production of proteid substances 

 accompanies carbon dioxide assimilation (Sect. 72). All these substances 

 are produced by the latter process, and are continually removed as plastic 

 or nutritive material to the different parts of the plant. Starch, sugar, 



and the other assimilatory products disap- 

 pear when carbon dioxide assimilation is 

 suspended, and reappear when it is resumed. 

 This fact was first observed and correctly 

 interpreted by Sachs l , who showed that after 

 twenty-four hours' darkness chlorophyllous 

 cells were in most cases devoid of starch, 

 and that the latter soon reappeared in light, 

 but only when carbonic acid gas was present 

 (Pfeffer, Godlewski). The fact that in an 

 atmosphere free from carbon dioxide starch 

 disappears in light or darkness (Fig. 48) 

 shows that the assimilatory products are 

 translocated from the leaf not only at night, 

 FIG. 48. Apparatus for growing plants but also in the daytime 2 . 



in an atmosphere free from carbon dioxide. . . , , . , 



Only those chloroplastids which are 

 directly illuminated assimilate carbon dioxide 

 and produce starch. Hence none of the latter 

 will appear in portions of a leaf which are 

 artificially darkened, so that by covering part 

 of its blade with a stencil plate or a sheet of 

 tinfoil, and subsequently treating the leaf with iodine after removing the 

 chlorophyll, a name may be printed on the leaf in bluish-black letters upon 

 a yellow ground (Fig. 49). 



Similarly, the formation of sugar may be shown to be dependent upon 

 the assimilation of carbon dioxide, as may be also the formation of plastic 

 proteid material in the leaf 2 . In order that assimilation may continue 



1 Sachs, Bot. Zeitnng, 1862, p. 368, and 1864, p. 289; Pfeffer, Monatsb. d. Berl. Akad., 1873, 

 p. 784; Godlewski, Flora, 1873, p. 382 ; also Morgen, Bot. Zeitung, 1877, pp. 553, &c. [See also 

 Costerus, Ann. d. Jard. bot. de Buitenzorg, T. xil, 1894, p. 73.] 



2 The fact that proteids are formed in the leaf does not necessarily show that they are produced 

 in the chloroplastids, or that they are primary assimilatory products, by whose decomposition 

 carbohydrates are formed ; nor is our knowledge of pyrenoids and crystalloids sufficient to answer 

 this question. Literature: Schmitz, Chromatophoren d. Algen, 1882, p. 150; A. Meyer, Bot. 



The bell-jar () is air-tight and is fixed upon 

 the glass plate (r), the tube (g) contains 

 pumice-stone moistened with potash to pre- 

 vent the entrance of CO 2 from without. 

 The vessel (s< also contains potash solution 

 to remove the CO^ given off from the soil 

 or from the plant. It may in some cases 

 be advisable to enclose a vessel containing 

 solid potash or CaCl_, in order that suffi- 

 cient transpiration may be possible. 



