VEGETABLE DYES 107 
in which the colour is not applied to the fibre from 
solution, but is developed within the fabric. 
The type from which this method of dyeing, known 
as “‘ vat dyeing,’’ was derived is the famous vegetable 
dye indigo blue. Indigo occurs in the form of a gluco- 
side, ‘‘ indican,” in the leaves and berries of a number 
of different plants. Sometimes it is associated with a 
yellow dye, and the plant yields a compound green. 
In England up to the sixteenth century the source of. 
indigo was the woad plant Isatis tinctoria. The plant 
was cultivated, and gave two crops of leaves in the year. 
The dye was developed by a long and tedious process, 
involving, first, a slow drying of the pulped-up leaves 
rolled into balls, then a damp fermentation lasting several 
weeks, and, finally, a second drying. Woad as a dye 
plant was superseded by imported indigo extracted 
from various Indigofera species cultivated in the tropics. 
But the woad industry still survived, on a small scale, 
up to the end of last century ; because the addition of 
small quantities of woad to the indigo vat was found 
to improve the fastness and penetrating power of the 
dye, and also to assist in the fermentation process used 
in its reduction. An interesting account of a woad 
mill at Parson Grove near Wisbech, where the primi- 
tive machinery and methods were still used in 1896, 
was given by Darwin and Meldola in Nature (1896). 
The cultivation of Indigofera in the tropics is, how- 
ever, a valuable industry and, as India is the chief 
producer, the maintenance of the natural dye is of direct 
imperial interest. 
In Bengal indigo land is ploughed in October, and 
the seed sown in drills the following February or March. 
