100 THE WORK OF PLANTS 
glass negatives, cleaned, are excellent), two pieces of 
cobalt chloride paper, and some geranium leaves en- 
tirely free from surface water. Dry the paper until it 
is blue. Place one piece of the paper on a glass plate ; 
place the geranium leaf with the underside on the paper. 
place the other cobalt paper, and next 
C ) the second piece of glass. On the 
Fra.135. Thestomateopen. narts well in contact. In fifteen or 
twenty minutes open and examine. T’he paper next 

a. On the upper side of the leaf now 
° pile place a light weight to keep the 
the underside of the geranium leaf 1s red where rt les 
under the leaf. The paper on the upper side is only 
slightly reddened. The greater loss of water, then, is 
through the underside of the geranium leaf. ‘This is 
true of a great many leaves, as tests which you can 
make will show. But it is not true of all. 
Why do many leaves lose more water through the 
underside? You will not be able to see with your 
eyes the mechanism in the leaf by which it can, to 
some extent, control the escape of the water vapour. 
It is too tiny. It can only be seen by using a micro- 
scope to look through pieces of the skin, or epidermis, 
of the leaf which we can strip off. Perhaps it will be 
just as well for the present if you look at a picture of 
it made from the leaf. Fig. 134 shows little holes 
through the epidermis. These open into spaces between 
O_O — —— a 
