Killing of Plant Tissue by Low Temperature 



253 



Freezing to Death of Pollen. The killing of the bloom and 

 young fruit of the peach, apple and some other fruits will be dis- 

 cussed later in this paper. However, it is perhaps not out of place 

 to discuss here the killing of pollen. Schaffnit^ exposed the pollen 

 of a number of species of plants to a temperature of -17 C. for eight 

 hours with no apparent harmful effects, while Sandsten^ apparently 

 reduced the germination percentage of pear, plum, cherry and peach 

 pollen by exposing for six hours to a temperature of 1.5. At this 

 station pollen of the Jonathan and Cillago apples was frozen with 

 the result shown in Table 37. The pollen was frozen for eighteen 

 hours and then germinated in a 10% sugar solution. 



Table 37. Showing Effect of Low Temperature on Germinat- 

 ing Power of Pollen Grains. 



Thus the pollen will still germinate after exposure to as low 

 temperature as -8 C., and dried pollen at as low a temperature as 

 -13, a much lower temperature than the other flower parts will with- 

 stand. 



Discussion of the killing temperature of other flower tissues 

 will be found in a later part of this paper. 



Rest Period of Plants. Closely associated with the cjuestion 

 of maturity in the fall is that of maintaining a condition of maturity 

 in late winter. Plants that have started into growth in spring soon 

 reach a condition whereby they are as tender as they were in early 

 fall, and sometimes even more so. Under conditions such as prevail 

 in the southern half of Missouri, where growth may continue latr in 

 autumn, and where in January and early Februar>-. there are likely 

 to be da>'s warm enough for coiisidcrablo growth to take place, it has 



'Mitt. Kalsor Wllhclm Inst. Landw HroinlMTK. Vol. :i. No. 2. pp. iKMl.1. IIUO. (Hll)l. 

 No. OS). 



-Wis. Agr. Exp. .Stu. Ucacarch Hul , pp liW-5 (HIbl. No. 07). 



