ELIMINATION 189 



nearly to the top of the metal shell or cover, in a vessel of chopped 

 ice, and cut off all radiation, by a woolen cover, from the ice to the 

 leaves. Stand the plant where its leaves will have the usual condi- 

 tions for three hours, then remove the pot, wipe dry the cover, and 

 weigh to determine the transpiration for that period. After leaving 

 the plant for an hour or two to recover, replace it in a vessel contain- 

 ing water which is heated by a spirit-lamp until the soil is at 35, 

 radiation to the leaves being prevented; keep it there for three hours 

 and determine the transpiration for that time. 



EXPERIMENT. Prepare a plant as for the preceding experiments, 

 but water it on successive days with water containing a harmless 

 salt, e.g., n/io potassium nitrate (about a i% solution), w/5, w/3, 

 and observe the effect upon the rate of transpiration. 



The student should now extend his knowledge of the quan- 

 titative phases of transpiration by a study of the subject in the 

 literature, giving especial attention to its quantities, the effect 

 produced upon it by external factors, its relation to evaporation; 

 and he should express these matters in a proper exposition. 



The student should turn next to a study of the transpiration 

 structures, of which he should review and extend his knowledge 

 until he understands, with a clearness permitting their accurate 

 diagrammatic representation, the construction of the leaf tissue, 

 as to connection of vessels with the mesophyl cells, the structure 

 of these, their connection with the intercellular spaces, the connec- 

 tion of the latter with the outside world, and the anatomy and mode 

 of action of the stomata and guard cells. 



Any study of leaf structure will suggest that the stomata 

 must play an indispensable role in transpiration, which presents 

 this important problem: 



What relation exists between transpiration, and the presence, 

 number, or condition of stomata? 



This may most readily be determined by applying a test of the 

 occurrence of transpiration to leaves, the distribution, number, and 

 open or closed condition of whose stomata is known. 



EXPERIMENT. Select from among the materials described later 

 under Stoma Quantities, leaves possessing (a) stomata on one side, 

 (b) on both sides but in unequal numbers, (c) on both sides but in equal 

 numbers, and (d) closed stomata (effected by partial wilting); apply 

 to them, by some form of leaf clasp, small discs of paper impregnated 

 with cobalt chloride dried to deep blueness over the Bunsen flame, and 

 observe the transpiration as indicated by the reddening of the papers. 



