154 MINNESOTA BOTANICAL STUDIES. 



By April 19th summer conditions were again established, and 

 the daily maximum temperatures for the next few days ranged 

 from 20° C to 26° C (68° to 80° F). On April 22nd the tem- 

 perature began to fall very rapidly, passed the freezing point 

 about five o'clock in the afternoon, reached a minimum tempera- 

 ture of -7.2° C (19° F) early in the morning of April 23rd and 

 remained below freezing until the morning of April 25th — over 

 sixty hours of continuous freezing. At the time of the lowest 

 temperature, on April 23rd, a northwesterly gale was blowing, so 

 that the temperature was very uniform and there were no pockets 

 of warm air as there sometimes are during cold but quiet nights. 

 This period of freezing, from April 22nd to 25th, will be spoken 

 of collectively as the "second frost." After April 25th the weather 

 became mild again, but was cooler than usual for about six 

 weeks and there were several light frosts after May 1st. 



It will be seen from this account that vegetation in a condi- 

 tion of active growth was subjected to two periods of prolonged 

 cold, the first with a minimum temperature of -1.7° C. the second 

 with a minimum temperature of -72° C. The eft'ects of the freez- 

 ing were noted in the case of a large number of species both 

 of woody and of herbaceous plants. 



Mechanical Injury Resulting From Lozv Temperatures. — In 

 both frosts it was found that much mechanical damage was done 

 to plants, often when they were otherwise quite frost-hardy. This 

 mechanical damage was mainly of two kinds. ( 1 ) . It is a 

 well-known fact that just before freezing occurs in plant tissue 

 much of the water of the cell sap is secreted into the inter- 

 cellular spaces. The resulting loss of turgidity allowed the young 

 and succulent twigs, often loaded with heavy foliage, to bend 

 sharply and in many cases the vascular tracts were unable to stand 

 the strain and breaks occurred. This damage was greatest in 

 the case of somewhat succulent herbaceous plants, such as peonies 

 and lilies and such native plants as Trillium grandiflorum. In 

 many cases, also, the young twigs of woody plants suffered in the 

 same manner. Sometimes when no actual break occurred the 

 bending was so great that the plant was later unable to straighten 

 the twigs and many curiously distorted shoots resulted. It was 

 estimated after the first frost that fully half the damage done to 



