lesz BOTANY 
eobalt chloride and dried in an oven till the red colour 
changes to blue. The papers should be held in place 
by sheets of glass clamped together. one above and one 
below (Fig. 91). The red colour showing that 
moisture is being absorbed will appear first in the 
paper applied to the lower surface of the leaf. 
Another simple experiment may be made by 
fastening two watch glasses by means of a clip opposite 
each other on either side of a growing leaf (Fig. 90), 
and sealing with vaseline the edge of the glasses where 
they meet the leaf surface. Drops of moisture soon 
form on the inside of the glass in contact with the 
lower surface of the leaf, while the glass on the upper 
side will be hardly dimmed. The position of the 
stomata on the under side is a great advantage, for 
it diminishes the chance of their being blocked by 
water or dust. 
The Control of transpiration is, in all plants where 
leaves are present, carried out by the adjustment of 
the stomata effected by the guard cells. When water 
is passing freely from the root, the guard cells become 
full of liquid and are said to be turgid. This has the 
effect of making them curve outwards and thus enlarge 
the stoma or opening through which the vapour passes. 
This curving is well seen in the filling of sausage skins, 
which, as they are filled assume a more or less crescent 
shape. When water is not being freely received by 
the leaf the guard cells become flaccid and assume a 
linear form go that, at the stoma, they come almost into 
contact in such a way as practically to close the 
opening. | 
Split the flowering stem of a dandelion or cat’s-ear 
longitudinally into long strips. Place these in alcohol 
and they will curve much in the same way as the tureid 
guard cells do. By osmosis, the alcohol passes 
through into the cells more quickly than the denser 
water passes out. As, moreover, the walls of the célls 
