80 PLANT PHYSIOLOGY 



soil may be made practically water proof by means of melted 

 paraffin whose melting point is sufficientl}'' 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 

 2-4-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, 

 remains fresh for a long time while the other two 

 wither quickly. 

 Fig. 42. (A 'p^g Icaves of the Cottonwood (Populus, vari- 



— R oot •\i ii-iVki 



pressure ous spccics) havc stomata on both sides. Repeat the 

 (Ji^f"™®'^^ 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 



