80 PLANT PHYSIOLOGY 
soil may be made practically water proof by means of melted 
paraffin whose melting point is sufficiently low so as not to 
injure the stem when applied to the top of the soil in a melted 
condition. Weigh the pot and place in a dry room for an hour 
and weigh again. Calculate the loss of water per square 
centimeter of leaf surface. Place in a moist room under the 
same light conditions as before and note the loss of weight in an 
hour. Such experiments are not accurate as many factors 
enter in to interfere, but they give an idea of the approximate 
amount of water evaporated. The experiment may be 
continued a long time by providing an opening in the rubber or 
paraffin through which a thistle tube passes and adding every 
twenty-four hours as much water as was lost in the preceding 
24-hour period. By keeping a record in this way, the amount 
of water lost in a week can be determined roughly. (Of course 
the increase in weight of the plant itself as it grows is a factor 
not taken into consideration in the foregoing nor the effect 
upon the roots of the partial exclusion of the air by the rubber or 
paraffin.) 
(k) To show that it is mainly through the stomata that 
evaporation (transpiration) occurs, take three lilac leaves of as 
nearly equal size as possible. Coat the ends of the petioles of 
each and the under surface of one and the upper surface of 
another leaf with a varnish made of equal parts of 
beeswax and lard or ordinary grafting wax if some- 
what softened. Both surfaces of the third leaf are 
to be left uncoated. The stomata are found only on 
the lower surface and it will be found that the leaf 
with this surface coated, thus covering the stomata, 
2 remains fresh for a long time while the other two 
“> wither quickly. 
__fis. 42, (1) The leaves of the cottonwood (Populus, vari- 
pressure ous species) have stomata on both sides. Repeat the 
experiment foregoing experiment with leaves of this and com- 
pare with the results obtained with the lilac. 
(m) Root pressure may be demonstrated by cutting off the 
stem of a rapidly growing sunflower or other rather large 
plant (e.g. tomato, geranium, castor bean, etc.) and slipping a 
heavy rubber tube over the cut stump, connecting this with a 
narrow glass tube. If the soil be kept warm and wet water will 
soon begin to escape from the cut surface and will rise to a 
