io6 
NATURE NOTES 
dissolved, turned into soluble sugar, and then transmitted 
through the plant to all parts which are growing, to be con- 
verted into cellulose, of which the cell-walls are composed. 
If more starch is made than is required for growth, the sugar 
can be reformed into starch in seeds, tubers, &c., as reservoirs 
for future use. Tincture of iodine touched on a cut bean or 
potato will at once reveal the starch stored up in the cells. 
As protoplasm must increase in quantity as the plant grows, 
another use of starch is as an ingredient in its manufacture ; but 
in this the element nitrogen (N) is required. This is supplied 
from certain mineral substances derived from the soil. 
If there be more of this manufactured “nitrogenous” sub- 
stance than required at the time, it is stored up with the starch 
in various ways. It may be distinguished under the microscope, 
for iodine colours it a yellowish brown ; but it is not so readily 
observable to the naked eye, though bran, which contains a 
great quantity, may reveal it. 
Transpiration. — This function consists of the throwing off 
the vapour of water, and is also an effect of light. Experi- 
ments made by placing plants under coloured glasses show that 
it is mainly a result of the red and violet rays ; at least maxima 
of transpiration occur with those bands of the spectrum. It is 
not due to heat-rays, though heat will cause evaporation of 
moisture from all wet substances, from living as well as from 
dead structures, if the heat be great. This, however, must be 
carefully distinguished from transpiration, which is a property 
of living protoplasm only. 
A simple experiment will show this. Take a tumbler full of 
water, place a card over it, and let a few leaves have their stalks, 
passing through a hole, standing within the water. Now cover 
them with a dry tumbler. Place it in the sun-light. In a few 
minutes the inside of the upper inverted tumbler will be covered 
with moisture. 
If, however, it be placed in total darkness, as in a cupboard, 
the temperature being the same, no moisture will be found. 
The importance of this function to the plant, of getting rid 
of superfluous moisture, resides in the fact that all plants 
require certain mineral salts ; but these are only dissolved in the 
minutest quantity in the water of the soils, say 2 per cent., so 
that to secure a sufficient supply of potash, lime, phosphoric acid, 
&c., for healthy growth, a great deal more water must be absorbed 
than the plant can possibly do with, and so it is exhaled or 
transpired. 
Absorption. — Leaves are not, as a rule, water-absorbing 
organs ; but if the loss by transpiration and evaporation be 
very great under brilliant sun-light and excessive heat, as in the 
African deserts, and, in fact, be greater than the supply by the 
roots, then they are quite able to take water in, cither from rain 
or dew. In fact, in some desert plants, as the tamarix, certain 
salts, especially “ chlorides,” are secreted by the leaf, which 
