30 



Elementary Plant Physiology. 



^ 



coating of ice on water in the open air. After four or five 

 hours note what portions of the shoot have been injured by the 

 exposure. Can any deterioration of the seeds be noted? Test 

 the two lots of seeds in a germinator, and ascertain if they have 

 suffered equal damage. 



Place a number of filaments of Spirogyra in a small dish 

 of water, and set in the open air during a night in which 

 the temperature is sufificient to freeze the water solidly. If 

 the experiment is to be performed during a warmer sea- 

 son, the freezing may be ac- 

 complished by a mixture of ice 

 and salt, or by the use of the 

 liquid carbonic-acid gas sold in 

 small steel cylinders by the 

 manufacturers. (See MacDou- 

 gal's " Practical Plant Physi- 

 ology " for preparation of freezing 

 mixtures.) Thaw the frozen ma- 

 terial, and examine in water under 

 a magnification of 400 to 600, and 

 note the changes produced in the 

 cells by freezing. 

 23. Influence of cold upon the rate of growth, — 

 Select two young plants showing rapid growth of the stems or 

 petioles, and cover with similar bell jars. Suspend thermome- 

 ters in the bell jars to indicate the temperature of the air around 

 the plants. Now remove the bell jars, and mark the organs to 

 be observed into centimeter intervals, as in § i . Set closely 

 around one plant a number of tall beakers containing freezing 

 mixtures or large pieces of ice, which should be renewed at 

 intervals of two hours. Read the temperatures in both bell 

 jars every hour for six hours, then remove the plants, and 

 measure the amount of growth in both plants. Replace under 

 the bell jars, and continue under the above conditions for 



Fig. 15. — Respiration tubes con- 

 taining seeds, and with tiie open 

 ends immersed in mercury. 



