METABOLISM 267 



with a microscope, the plastids will be found in the cells, 

 but they will be tinged with a pale yellow pigment known 

 as etiolin. When the latter is exposed to light it will 

 rapidly become green, being in fact converted into chloro- 

 phyll. The etiolin is in the first instance secreted by the 

 protoplasm of the plastid, and subsequent changes take 

 place about which very little is known, but which result in 

 its conversion into chlorophyll. If the temperature is kept 

 very low, the etiolin remains unchanged, even though light 

 is admitted. Hence the first leaves of plants which spring 

 up in winter or early spring are frequently yellow and not 

 green. This peculiarity may easily be observed in the case 

 of snowdrops and hyacinths which appear very early in the 

 year. 



The function of the iron is not understood ; plants which 

 are cultivated in such a medium that this element is not 

 supplied to them have an appearance much like that 

 associated with etiolation. Their colour is even paler, 

 indeed they are almost colourless, though the plastids are 

 present. A supply of iron at once causes them to assume 

 the normal appearance. Plants so suffering from the 

 absence of iron are said to be chlorotic. 



The influence of a supply of oxygen is probably not 

 a direct one. The failure of plants to form chlorophyll in 

 its absence is most likely due to a pathological or unhealthy 

 condition of the protoplasm, all whose activities are dis- 

 turbed under such circumstances. 



Another pigment which is of fairly widespread distri- 

 bution in plants is the red colouring matter known as 

 anthocyan. This is not associated with any plastids, but 

 occurs in solution in the cell-sap. It is found very com- 

 monly in young developing shoots, on the illuminated side 

 of leaves which appear during cold weather, on the petioles 

 and midribs of leaves which are put out on twigs of many 

 plants in sunny places, and in many tropical plants which 

 grow in deep shade. In seedlings which are developed in 

 spring or in cold weather, the anthocyan may appear some- 



