1380 SOUTH ‘AFRICAN BOTANY 
layer of sand on marble. The marble must be perfectly 
smooth, and the sand must be kept moist. In a few 
weeks when the seeds have grown well, if the sand is 
washed off fine markings will be seen in the marble— 
these are caused by the acid from the roots dissolving 
away the marble just where they have touched it. 
97. Transpiration.— We saw in the experiments on 
water culture that the solutions were very dilute,i.e. the 
root absorbs a very large quantity of water in order to 
obtain a comparatively small amount of salts. Much 
of this water actually enters into the composition of 
the plant, some plants containing as much as 90 per cent 
of water; but a great deal of it is not wanted, and is 
therefore given off by the leaves. This escape of water 
vapour from a plant is called TRANSPIRATION, and the 
current of water which passes from roots to leaves is 
called the Transpiration current. 
98. Experiments in Transpiration :— 
Experiment 7.—To show that leaves give out water 
vapour. 
Method.—Fix some long-stalked leaves through a 
card and seal up the opening. Place the card over a 
tumbler of water so that the stalks dip into water, and 
invert a clean dry beaker over the leaves. 
Result.—In a short time drops of water are seen on 
the glass. 
Conclusion.—The leaves have given off water vapour, 
which has condensed on the glass. 
Experiment 8.—To see which surface of the leaf gives 
off moisture most quickly. 
Apparatus.—Filter papers, cobalt chloride. 
