GENERAL OBSERVATIONS ON FREEZING 



95 



One has received the customary low temperatures during the 

 resting period, and the other has been kept abnormally warm. 

 The best results will be secured with some species native to the 

 region in which the test is performed. 



147. Freezing of Unicellular Organisms. Mount a number of 

 healthy filaments of Spirogyra on a glass slip and make examina- 

 tions, and exact drawings of a 



few cells. If the test is made in 

 midwinter the slide may be placed 

 outside on the window sill for 

 half an hour, and allowed to 

 freeze. If made at other times 

 or places, freezing mixtures of ice 

 and salt may be employed,, or 

 still better the slide may be sub- 

 jected to the action of escaping 

 liquid carbonic acid, which will 

 give instant and low tempera- 

 tures. Bring the slide frozen by 

 any of these methods into the 

 laboratory, and allow it to thaw 

 gradually. Keep under constant 

 examination, and determine 

 whether ice crystals are actually 

 formed inside the cells or not. 

 Note effects on cell and draw. 



148. The Freezing of Tissues. 

 Mount a leaf of Philotria on a 



\. , r after thawing. After Molisch. 



glass slip and expose to freezing 



temperatures as above, and note results. Repeat using sections 



of stems containing living parenchymatous and embryonic 



tissues. 1 



149. General Observations on Freezing. The actual shock to a 

 protoplast by freezing appears to be accompanied by the with- 



1 Molisch, H. Untersuchungen ueber das Erfrieren der Pflanzen. Jena. 1897. 



a > c 



FIG. 42. Spirogyra. a, normal, b, 

 frozen and imbedded in ice. c, same 



