MORPHOLOGY OF HIGHER PLANTS. 351 



follows : On the day previous to the demonstration of the experi- 

 ment the leaf shield is clamped on the leaf. On the following day 

 the leaf is allowed to stand well exposed to the sunhglu for 

 several hours. The leaf is then removed from the plant, separated 

 from the leaf shield, and after the chlorophyll is extracted hy 

 boiling in alcohol it is placed in weak iodine solution, whereby the 

 entire part of the leaf exposed to the light is darkened in color, 

 while the area protected by the tinfoil remains unchanged. 



The phenomenon of photosynthesis whereby the leaf uses 

 carbon dioxide and gives off oxygen can be demonstrated by means 

 of a photosynthometer. Ganong gives a detailed description of 

 its construction and use. The underlying principle is that a 

 known volume of leaf material is supplied with a known volume 

 of carbon dioxide. After submitting the leaf to the sunlight a 

 test is made of the proportion of carbon dioxide and oxygen in the 

 graduated tube, from which the activity of the leaf may be 

 accurately deduced. 



The process of transpiration in leaves can also be readily 

 shown. The mere fact of transpiration can be shown by inserting 

 the leaves of a plant into a bell-jar which has at its base a sheet 

 of rubber or other suitable material stretched over the opening 

 with just sufficient aperture to admit the stem. The root being 

 outside of the bell-jar can be kept moist and as transpiration 

 takes place the moisture condenses on the inside of the glass. 



By means of an arm balance and devices for measuring the 

 water supplied the plant, together with some covering to prevent 

 evaporation from the surface of the soil, rather accurate deter- 

 minations can be made of the quantities of water given off by the 

 plant and calculations made for each square unit of leaf surface. 



The potometer is another ingenious simple contrivance which 

 measures most accurately the amount of water transpired. It 

 consists of a horizontally placed graduated tube wliich ends in a 

 short vertical portion into which a stem bearing leaves can be 

 tightly inserted after the instrument is filled with water. By 

 means of a reservoir at the side and a stop-cock more water can 

 be admitted as needed. Accurate readings over short periods of 

 time are facilitated by admitting a small bubble of air as an index 

 and observing the time of its passage along the graduated tube. 



(Consult Ganong: Plant Physiology.) 



