240 LECTURE XII. 



that the absence of iron not only prevents the formation of 

 chlorophyll, but even the differentiation of the corpuscles 

 (see p. 136). The recent researches of Schimper and of 

 A. Mayer prove, however, that this view is quite untenable. 



Plants, then, which are normally green, are not green if 

 they have been grown in the dark, or if the temperature 

 has been too low, or if they have not been supplied with 

 iron : but the resulting colour is not the same in all these 

 cases. A plant which has been growing in the dark or at 

 a low temperature is usually of a yellow colour ; a colouring- 

 matter has been formed in it, but instead of being a green 

 it is a yellow colouring- matter termed etiolin. A normally 

 green plant which has been grown in the dark is said to be 

 etiolated. On the other hand, a plant which has been deprived 

 of a supply of iron becomes perfectly colourless, and is said 

 to be chlorotic. When an etiolated plant is exposed to light 

 and the temperature is sufficiently high it rapidly becomes 

 green : similarly when a chlorotic plant is supplied with iron 

 it also becomes green. 



It appears that the formation of chlorophyll in the plant 

 under ordinary conditions is mediate, that is, that it is not 

 directly formed, but that etiolin is first formed, and that 

 from etiolin chlorophyll is produced. In endeavouring to 

 ascertain the mode of formation of chlorophyll, the first step 

 will be to enquire into the origin of etiolin, and the second, 

 to determine the nature of the process by which etiolin 

 is converted into chlorophyll. 



The process of the formation of etiolin appears, from the 

 researches of Gris, of Mikosch, and others, to be as follows. 

 When the protoplasmic corpuscle is fully formed, it produces 

 a starch-grain in the manner described in the case of the 

 amyloplasts in a previous lecture (p. 180); it then gradually 

 assumes a yellow colour, and, at the same time, the included 

 starch-grain diminishes in size and finally disappears ; if now 

 the etiolin-corpuscle, as it may be termed, is exposed to 

 light, it assumes a green colour, the etiolin being converted 

 into chlorophyll. 



With regard to the chemical nature of this process, 



