102 EXPLOITATION OF PLANTS 
The colouring matters which serve the dyer are 
generally present in the plant in the form of soluble 
colourless compounds with various sugars, called 
glucosides. The conditions governing their produc- 
tion are only vaguely understood ; but it seems prob- 
able that they are waste products, which the plant has 
no means of excreting, They are found, as a rule, in 
the bark of -stems or tubers, or in the leaves. The 
number of different plant dyes is surprisingly small, 
but each of the principal elementary compounds has a 
wide range in the vegetable world. Tropical plants, 
on the whole, produce larger quantities of the same dye 
than the plants of temperate regions. Beyond this, 
and the fact that the Leguminose provide a large pro- 
portion of the world’s dye plants, it is impossible to 
generalise. 
The object of the dyer is to impart a permanent 
colour to his fabrics. For this purpose the colouring 
matter must be firmly united to the fibres. It is a dis- 
puted point whether, in dyeing, this is due to an actual 
chemical combination between the molecules of dye and 
fabric, or whether it is a physical property of the large 
colloidal particles of the material, known as “‘ adsorp- 
tion.”” Probably, in most cases, the two effects are 
combined and, when the processes of dyeing are slowly 
performed, a purely physical adsorption is gradually 
followed by chemical combination. 
The fabrics may be sharply divided into two classes, 
according to their origin from animal or vegetable 
sources (mineral fibres, like spun glass or asbestos, 
cannot be dyed). 
The animal fabrics, wool, silk, leather, feathers, etc., 
