82 PLANT PHYSIOLOGY. 



exposed are very different. The effect of air temperature upon 

 transpiration has been found in Experiment 20 ; we have next 

 to ascertain 



42. How is the rate of transpiration affected by changes in 

 soil-temperature ? 



Answer by Experiment 2 i . 



EXPERIMENT 21. To test this, the soil temperature of a plant 

 must be varied independently of that of the top, which may be done 

 by placing the roots of a growing plant in a glass jar, the temperature 

 of which can be raised and lowered while radiation from it to the 

 shoot is cut off by a woollen covering between. Prepare a plant as 

 shown in Fig. 14 (on the right). Thrust a thermometer through the 

 rubber into the soil, and hang another in the air by the shoot on a 

 stick thrust through the rubber. Weigh ; then cool the glass jar to 

 about 5 by immersing it in water containing melting ice or snow ; 

 keep it at 5 three hours, then remove the jar, dry it, and weigh. 

 Later warm the jar by slowly warming the water with a spirit-lamp 

 until it reaches 38 to 40; keep it there for three hours, and weigh. 

 In both cases cover the jar with a woollen wrap and set the whole 

 in an ordinarily favorable situation. 

 The importance of this experiment consists in the light it throws upon 



the ecological significance of the occurrence of xerophytic characters in 



many hydrophytic plants. 



43. What is the construction of the Leaf as a transpiring 

 structure ? 



Answer by a review of your knowledge of leaf-structure, 

 especially including the stomata. Construct a simple diagram 

 of the leaf as a transpiring structure, bringing out the path of 

 the water from the ducts to the air outside of the leaf. Explain 

 by diagrams the working of the stomata. 



44. Are stomata indispensable to transpiration in the higher 

 land plants? 



Answer by Experiment 22. 



EXPERIMENT 22. This may be answered by observation of the 

 relative amount of transpiration from the two sides of a leaf in 

 which one side has stomata and the other none. Select such a 

 plant, e.g., Ficus elastica ; place upon the two sides watch-crystals 

 held in place by a spring or similar device. Set the leaf vertically 

 so that both sides may receive about equal illumination, and observe 



