RESISTANCE OF PLANTS 359 



perature, about 0°C. or even somewhat higher, respire intensely, 

 utilizing all the oxygen at their disposal. The thick snow cover 

 inhibits the access of fresh air, and the plants suffocate. How- 

 ever, recent investigations (Tumanov, 1932) have shown that 

 even under a thick layer of snow the air in direct contact with 

 the plants contains sufficient oxygen and that the cause of their 

 death is not suffocation but exhaustion of reserves. Remaining 

 at a temperature about 0°C., when respiration is sufficiently 

 intense, and being at the same time in almost complete darkness, 

 which precludes the possibility of photosynthesis, the plants 

 gradually exhaust all their reserves of nutritive substances and 

 finally die of starvation. 



Resistance to exhaustion is conditioned by peculiarities other 

 than resistance to freezing, and thus hardy varieties are not 

 always capable of enduring prolonged periods under a thick snow 

 cover. A low respiratory intensity at low temperatures plays 

 an important role in resistance to fungi; but as in the case of 

 hardening, the accumulation of large carbohydrate reserves is 

 likewise of great significance. 



Another danger, which must not be confused with exhaustion, 

 is injury from an excess of water. It is frequently observed in 

 spring during prolonged periods of thawing, when melted water 

 accumulates on the surface of the frozen soil, and floods the 

 plants. In this case, the death of the plants is a result of suffo- 

 cation from oxygen deficiency. In the snow water surrounding 

 the plants, alcohol, the chief product of anaerobic respiration, 

 may be detected. If with the return of cold weather this melted 

 water freezes again, forming an ice crust entirely covering the 

 plants, the latter, being previously weakened by suffocation, are 

 almost sure to die. Ice crust is one of the frequent causes of 

 death of winter cereals in large areas,^ especially in the southern 

 part of Western Russia. 



If the melted water has been absorbed by the soil, a new danger 

 arises with the subsequent freezing of the soil, viz., heaving, 

 which breaks the roots of the plants. The freezing of the soil 

 water begins very often not from the surface but at some depth 

 on the boundary of the soil layers that have not thawed. An 

 ice layer is formed that thickens gradually and lifts the upper 

 soil layers together with the plants, thus pulling them out of 

 the soil. The roots that in autumn have penetrated to a con- 



