1914] LAND—TEMPERATURE CONTROL 521 
sections are required (3-5 uw), paraffin melting at 58-62°C. is 
necessary. Even when the harder paraffin is used, in order to cut 
sections 2-4m thick, at ordinary room temperature, the knife 
and block must be cooled. If they are not cooled below the room 
temperature, which in winter is usually about 20-22° C., the sec- 
tions are hopelessly crushed. In summer it becomes increasingly 
difhcult to make thin sections. 
Fic, 1.—Apparatus for temperature control, showing object holder and cooling 
trough in place. 
Many delicate plant tissues, notably certain liverworts, can be 
imbedded in paraffin melting at 45-52° C. with perceptibly little 
injury, but when the same material is imbedded at a temperature 
of 58-62° C. serious injury may result. For example, a tropical 
species of Notothylas, which at 52° showed little shrinkage, at 62° 
shrunk to neatly half the origina] thickness of the thallus. In 
addition to the action of heat, the effect of the large coefficient of 
expansion of paraffin (0.00027854) becomes marked when the 
harder paraffin is used. It follows that by using paraffin of a low 
melting point, the effect of heat and of the coefficient of expansion 
can be minimized. 
