THE FOOD OF PLANTS. 9 



2. The Production of Organic Substance in Green Plant Cells 

 under the Influence of Light. 



The production of organic material in the green plant cell is 

 dependent on the co-operation of light, an important doctrine in 

 plant physiology, of the truth of which we must satisfy ourselves 

 by experiment. We weigh separately a few maize grains, and 

 estimate their dry weight. (See 1.) After soaking the grains 

 and germinating them in sawdust, each seedling is separately 

 fixed, in the manner described in 1, in a cylinder filled with food 

 solution. Some of the culture vessels are now placed in darkness 

 under a large cardboard box ; the others, under otherwise similar 

 conditions, are exposed in a very well lighted place to the alterna- 

 tion of day and night. The leaves of the plants kept in the dark 

 do not become green like those of the illuminated plants, but 

 assume a yellow colour, since the normal green chlorophyll pig- 

 ment can only develop in the cells when they are exposed to the 

 light. After four or five weeks we remove the plants from the 

 culture solutions, dry them in the air, and determine the weight 

 of dry substance in each individual, which weight is then to be 

 compared with that of the corresponding seed grain. The dry 

 weight of the illuminated plants is found to be considerably 

 greater than that of the maize grains started with, while the dry 

 weight of the plants grown in the dark, is, as I ascertained, about 

 50 per cent, less than that of the seeds. In absence of light no for- 

 mation of new organic material can take place, but on the contrary 

 a large portion of the organic bodies already present are broken 

 down in metabolism (respiration). In presence of light it is true 

 the breaking down of organic material in respiration also takes 

 place, but the losses due to this are more than covered by assimi- 

 lation, so that the dry weight of plants vegetating in the light 

 progressively increases.* 



3. The Organs of Assimilation. 



If we consider in the first place only what is presented to us 

 in the higher plants, it is to be noted that most of them have 

 well developed foliage leaves. These are to be placed in the very 

 first rank as organs of assimilation. Their blade presents a large 



* For similar experiments see Detmer, Versuchsstationen, Bd. 14. 



