160 PRACTICAL COURSE IN BOTANY 



ni. TRANSPIRATION 



Material. — Leafy twigs of actively growing young plants. Sun- 

 flower, corn, peach, grape, calla, and arums in general transpire rapidly; 

 thick-leaved evergreens and hairy or rough species, like mullein and hore- 

 hound more slowly. For Exp. 63, small-leaved, large-leaved, and thick- 

 leaved kinds will be needed. 



Appliances. — Glass jars and bottles with air-tight stoppers ; a little 

 vaseline, oil, gardener's wax, thread, cardboard, and a pair of scales. 



Experiment 62. To show why leaves wither. — Dry two self- 

 sealing jars thoroughly, by holding them over a stove or a lighted lamp 

 for a short time to prevent "sweating." Place in one a freshly cut leafy 

 sprig of any kind, leaving the other empty. Seal both jars and set them 

 in the shade. Place beside them, but without covering of any kind, a 

 twig similar to the one in the jar. Both twigs should have been cut at 

 the same time, and their cut ends covered with wax or vaseline, to prevent 

 access of air. Look at intervals to sec if there is any moisture deposited 

 on the inside of either jar. If there is none, set them both in a refrigerator 

 or cover with a wet cloth and allow to cool for half an hour, and then ex- 

 amine again. In which jar is there a greater deposit of dew ? How do you 

 account for it ? Take the twig out of the jar and compare its leaves with 

 those of the one left outside ; which have withered the more, and why ? 



Experiment 63. To measure the rate at which water is 



GIVEN OFF BY LEAVES OF DIFFERENT KINDS. — Fill three glaSS VCSSCls of 



the same size with water and cover with oil to prevent evaporation. 

 Insert into one the end of a healthy twig of peach or cherry; into the 

 second a twig of catalpa, grape, or any large-leaved plant, and into 

 the third, one of magnolia, holly, or other thick-leaved evergreen, letting 

 the stems of all reach well down into the water. Care must be taken to 

 select twigs of approximately the same size and age, since the absorbent 

 properties of very young stems are more injured by cutting and exposure 

 than those of older ones. All specimens should be cut under water as 

 directed in Exp. 58. Weigh all three vessels, and at the end of twenty- 

 four hours, weigh again, taking note of the quantity of liquid that has dis- 

 appeared from each glass. This will represent approximately the amount 

 absorbed by the leaves from the twigs to replace that given off. Which 

 twig has lost most? Which least? Note the condition of the leaves 

 on the different twigs; have they all al)sorl)ed water about as rapidly 

 as they have lost it ? How do you know this ? Pluck the leaves from 

 each twig, one by one, lay them on a flat surface that has been previously 

 measured off, into square inches or centimeters, and thus form a rough 

 pstiiflate of the area covere4 by each specinjen. M^kc the best estimate 



