22 TEXTBOOK OF PLANT PHYSIOLOGY 



oxygen formed in assimilation, taking it from the peroxide-like 

 combination of chlorophyll with carbon dioxide (see Art. 9), 

 which forms during the early stages, and afterwards giving it 

 up to the atmosphere, thus promoting photosynthesis. These 

 theories, however, have not been sufficiently established. 



The green color of the chlorophyll is always present in the plas- 

 tids of the blue-green, red, and brown algae, but it is disguised 

 by the presence of special pigments. As these are soluble in water, 

 they may be washed out from the dead cells of these plants, which 

 then turn green. The bluish color of the Cyanophyceae is produced 

 by phycocyanin. The color of the red algae is due to phycoery- 

 thrin, a pigment showing many modifications, wherefore the red 

 algae are of different hues, while the color of the brown algae is 

 induced by the presence of fucoxanthin. Our knowledge of the 

 chemical nature and the physiological importance of these addi- 

 tional pigments in the algae is still very meager. Some authors, 

 like Engelmann, assign to them an active part in the process of 

 assimilation. This will be discussed in Chap. VIII. 



7. Conditions of Chlorophyll Formation. Etiolation and 

 Chlorosis. — The formation of chlorophyll is very little understood 

 at present. It is known, however, that the process is conditioned 

 by several quite definite requirements. Of these one of the most 

 important is the presence in the cells of plastids, which can 

 change to a green color. Chlorophyll is never formed in the 

 roots, excepting in plants that have aerial roots, such as tropical 

 orchids. Another important requirement for the formation of 

 chlorophyll is light. Leaves and stems developed in darkness 

 remain almost white or are colored yellow by carotin and xantho- 

 phyll. Such plants are said to be etiolated. They turn green 

 rapidly when exposed to light. It is supposed that they contain 

 a special substance called "leucophyll," which is colorless in itself, 

 but readily changes into chlorophyll under the influence of light. 

 The nature of this chromogen (as substances easily turning into 

 pigments are called) has not been investigated. In some cases, 

 for instance in the germinating seeds of conifers, chlorophyll forms 

 also in the dark. 



Beside their pale hue, etiolated plants differ from normal ones 

 also in respect to their form. The aerial organs usually grow out 

 very long and may be somewhat thickened, while the leaves, on 

 the contrary, are undeveloped (Figs. 7 and 8). This modification 



