HOW PLANTS FEED 29 
carbon-dioxide in air. But the gain of weight in consequence 
of the absorbed carbon is so small in comparison with the 
loss brought about by transpiration as to be inappreciable, 
and may be neglected. Hence the difference in weight may 
be taken as representing the loss by transpiration. 
(b) Place a stem bearing leaves in a jar of water as 
shown, fixing the stem firmly in the cork and allowing the 
cut end to enter the water. The cork must fit well and 
the hole through which the stem passes must be made 
water-tight to prevent the passage of vapour from the 
water in the jar. This can be done by pouring a little 
melted grease round the hole, after the stem has been 
inserted. | 
In the centre of a piece of cardboard of the necessary 
size cut a hole so that the cardboard will pass over the 
neck and rest on the shoulder of the jar. 
This will support a tumbler as shown in 
fig. 25, which must have its rim well- 
greased to make a vapour-tight joint. 
When this arrangement is placed in the 
sunlight, drops of water soon collect on 
the inner surface of the inverted tumbler, 
showing that water must have been given 
off in the form of vapour from the leaves. 
Uses of Transpiration, The amount 
of plant food held in solution in the water 
which passes into the roots is very 
small ; hence it becomes necessary for the plant to absorb a 
relatively large amount of water, in order to obtain a supply 
of food sufficient to support its growth. When the water 
has travelled up to the factories of the plant—the leaves — 
and has done its work of carrying up the salt solutions, 
it is got rid of to make room for fresh supplies. That is, 
transpiration enables the plant to obtain weak salt 
