168 EXPLOITATION OF PLANTS 
source. Yet, were the source of each such substance 
discovered, there would open up a vista of potential 
economies, and we might then use coal only to get from 
it the things not obtainable in other ways. To-day 
Americans are finding it pays to grow sissal (Agave 
rigida) for the production of alcohol from its leaves, 
and the necessary machinery is run by the dried and 
caked fragments of the same leaves used as fuel. Both 
in German East Africa and British East Africa sissal 
covers hundreds of thousands of acres, and, it is said, 
the profit in exploiting it thus amounts to 20 per cent. 
at least. But this is merely one illustration of illimit- 
able possibilities. The French, sadly out of coal by 
the war, have formed a committee to look into practical 
gas manufacture from substitutes for coal. To what 
do they turn to supplement the power they owe to coal ? 
Again to plants. In a recently published report they 
found that even the squeezed pulp residues from olives 
gave not only a gas with a high illuminating power, 
but a charcoal which, with tar, makes good briquets for 
fuel, From a péculiarly resinous material, the strainings 
of the resin tapped from pine trees, they got a gas of 
specially high illuminating power. 
Though at present there is no attempt to correlate 
these products with the details of botanical structure 
of the plants thus used or usable, such research cries out 
to be done; some day it will be done. 
When we thoroughly know our coals, their contents 
and the sources of their contents, we shall have gone a 
long way toward overcoming the dangers of lack or 
restriction of coal. I see, as no impossible dream, not 
only the wise utilisation of the fuel we have, but the 
