SECRETION AND EXCRETION 463 



will be found in the cells, but they wiU be tinged with a pale 

 yellow pigment known as etiolin. When the etiolin is ex- 

 posed to light it will rapidly become green, being in fact re- 

 placed by or converted into chlorophyll. The etiolin is in the 

 first instance secreted by the protoplasm of the plastid, and sub- 

 sequent changes take place about which little is known, con- 

 verting it into chlorophyll. If the temperature be kept very 

 low, the etiolin remains unchanged, even though light be 

 admitted. Hence the first leaves of plants which spring up in 

 the winter are frequently yellow and not green. The function 

 of the iron is not understood ; plants cultivated in such a 

 medium that this element is not supplied to them have an ap- 

 pearance much like that of an etiolated plant. Their colour is 

 even paler, indeed they are almost colourless, though the 

 plastids are present. A supply of iron at once changes them 

 to the normal appearance. Plants so suffering from the absence 

 of iron are said to be chlorotio. 



The excretions of plants, using the term in the wide sense 

 indicated above, and not implying that they are thrown off 

 from the plant body, are similarly produced. Perhaps the most 

 frequently occurring instance of these is the sugary solution 

 known as the nectar, which is so common in flowers. Other 

 instances are the resin produced in the resin glands of the 

 Conifers and of other trees, and the etherial oils found in 

 special receptacles in the leaves and other parts. Mineral 

 matters, chiefly carbonate and oxalate of calcium, are also 

 formed. The carbonate is in some cases excreted on to the 

 surface through special glands, as in certain Saxifrages ; in 

 others it is deposited in the substance of the cell-walls or of 

 protrusions from them, as in the oystohths of Ficus, the Nettle, 

 and other plants. The oxalate is frequently deposited in 

 special cells, where it forms the bodies described as raphides 

 and sphseraphides. In these cases the cluster of crystals is 

 usually invested by a delicate skin derived from the protoplasm. 

 The oxalate is also sometimes deposited in the substance of the 

 cell-wall as in the bast fibres of Ephedra. Silica, again, is 

 accumulated in the epidermis of many grasses and Equisetums. 



Blany of these excretions cannot be traced to the self-decom- 

 position of the protoplasm,- but are probably formed by the 

 processes of oxidation and reduction which we have seen are 

 often associated with its activity. They are not, however, 

 formed without its intervention in some way. 



The bye-products of metabolism' are too numerous to be 



