METABOLISM 273 



the resulting seedling will not be green, but will have a 

 yellowish-white colour. When its tissues are examined 

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

 but they will be tinged with a pale yellow pigment known 

 as etiolin. This 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 replacement by 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. They are' indeed almost colour- 

 less, 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 or purple colouring matter known 

 as anthocyan. This is not associated with any plastids, but 

 occurs in solution in the cell-sap, particularly of the cells of 

 petals and some other parts of flowers ; it is found also very 

 commonly 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 



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