ADJUSTMENT TO LIGHT 81 



of an etiolin-like substance in them is unknown. The yellowish 

 or colorless plants produced by darkness are said to be etiolated. 

 Etiolation affects not only the color of the plant, but its form 

 and structure as well. Stems usually become thin and elongated, 

 and their branching is reduced. The size and number of the leaves 

 are decreased, and the latter are often reduced to mere scales. 

 There is a corresponding reduction in their structure, the dis- 

 tinction between palisade and sponge tissues disappearing as a 

 rule. 



Experiment 23. Influence of light and darkness. Plant sunflower 

 seeds in four small pots. Place two pots in the dark, and leave two in 

 the light. After the seedlings have grown to a few inches, exchange a 

 green plant and a colorless one. After a few days note the behavior 

 of the chlorophyll. Make a drawing of the plant that has been con- 

 stantly in the dark, and of the one kept in the light. Make a cross- 

 section of the stem and leaf of each, and illustrate the important differ- 

 ences by a drawing. 



ID I. Photosynthesis. The decomposition of carbon dioxide 

 and water by the action of light, and the combination of the products 

 into sugars, is termed photosynthesis. It is the special duty of 

 the chloroplast, and can take place only in light. It is most 

 active in sunlight, decreasing with a decrease of light intensity, 

 though at a varying rate for different species. For flowering 

 plants, photosynthesis becomes practically impossible, even for 

 shade forms, when the light intensity reaches .001. In the case 

 of many sun plants, the minimum is reached long before this, 

 depending upon the stability of the species concerned. In nature 

 photosynthesis is always due to the presence of white light. By 

 experiment it may be shown that certain colors of the spectrum 

 are more active than others in this process. It is found that the 

 greatest activity occurs in orange-red, somewhat less activity in 

 the blue, and little or none in other portions. These regions are 

 those in which the absorption of light by the chlorophyll is greatest, 

 a fact which gives support to the view that it is this absorption 

 which furnishes the energy necessary for photosynthesis. 



102. Absorption and diffusion of carbon dioxide. Photo- 

 synthesis depends, moreover, upon the presence of the crude 

 materials with which it is concerned, viz., carbon dioxide and 

 water. We have already seen how the water absorbed by the roots 



