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 toa 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- 
