May, 1921. 



SCIENTIFIC AGRICULTURE 



209 



"Hardiness" in Plants Which Live in the Open 



Over Winter 



\iy L. H. NEWMAN. 



Secretary, Canadian Seed Growers' Association, 



Ottawa. 



The question of "hardiiies.s" in plants which are re- 

 quired to survive the exacting conditions of winter and 

 early spring in the northern sections of North Am- 

 erica is one which has interested both the scientist and 

 the practical man for years. While much data of scien- 

 tific and practical value has been recorded, thi§ has 

 been interpreted in different ways, altliongh the gen- 

 eral concensus of opinion seems to be that "hardiness" 

 in itself is a complex character and one which is in- 

 fluenced very greatly by cultural methods. Investi- 

 gations seem to show further that different sorts are 

 better equipped to withstand frost than are others, in 

 fact it .seems clear that 'hardiness' is what is called a 

 'sort character". This being the case, the .study of 

 hardiness mu.st take into consideration both the breeding 

 and the feeding of the plant. The latter phase has been 

 dealt with in a particularly pleasing manner by Dr. 

 lledlund, Biologist at the Agricultural College at Al- 

 narp, .Sweden, who has shown in detail just how the 

 food supply acts in preparing the plant to resist frost. * 

 While this account contains very little that is new to 

 physiologists, the manner of its presentation and the 

 suggestions which are offered are such as to provoke 

 considerable thought. It is explained by Dr. lledlund 

 at the outset that in tlie case of those jtlants which must 

 remain outside during the winter, the damage which 

 they may suffer from frost is due largely to the forma- 

 tion of ice within the cell tissue. That it is not the 

 formation of ice in itself which causes the death of the 

 protoplasm in the cell is .shown by the following simple 

 experiment which he cites as follows : 



"If a stiffly frozen cabbage is taken into a warm room 

 where it thaws out quickly, the leaves appear lax and 

 withered. The protoplasm in the cells has been killed. 

 The membrane which covers it has lost its semi-perme- 

 able quality and can no longer retain tlie soluble sub- 

 stances within the cell." This results in a decreased 

 osmotic pressure, that is the pressure exerted as a result 

 of the tendency of fluids of different kinds and densi- 

 ties to become diffused through a separating membrane 

 when placed in connection with it. If the same cabbage 

 were allowed to stand outside after it had thawed out, 

 it might be found to be fresh and living. FVom this it 

 may be concluded that it was not dead but had stood 

 frozen, death occui-ing only when thawing took place too 

 suddenly. Too sudden freezing was also shown to have 

 the same effect. Many plants may be frozen .stiff with- 

 out injury, if they are only allowed to freeze and thaw 

 slowly. Tliey cannot however, stand an unlimited de- 

 gree of frost. If the temperature sinks very low — dif- 

 ferent for different plants — the protopla.sm in the cells 

 of course dies, even thongh thawing takes place slowly. 



In order to show the essential cause of the death of a 

 plant by freezing, the above writer shows exactly what 

 takes place within a plant which has been frozen to such 

 an extent as to cause its death. He savs: "When a living 



•T. Hedlund: "Om Frosthardighten hos vara kall- 

 jordsvaxter, Svensk Botanisk Tidskrift, 1912, Bd. 6. 

 h. 3. 



plant is allowed to freeze graduall.v, ice is formed in 

 the intercellular spaces. The water for the formation 

 of this ice is taken from within the cell, passing through 

 the cell wall and forming ice on the outside of the same. 

 The surface or area which is covered with ice is now 

 greatly increased. The formation of ice can often be- 

 come so great that the cells become separated from each 

 other for a considerable distance. A misroscopical 

 examination of the young bark of trees and bushes in 

 the spring after a severe winter .shows that this, on its 

 inside, is often split in long fissures parallel with the 

 surfaces. A plant does not seem to be damaged however 

 by this splitting. It is therefore necessary to investigate 

 the changes which take place within the protoplasm as 

 a result of the formation of ice. As mentioned above, 

 when ice is formed between the cells the water out of 

 which it is formed was taken from within the cell. As 

 a result of this loss of water, the cell walls arc no longer 

 distended. The volume or space within the cell being 

 lessened by the relaxation of the cell walls, causes the 

 protoplasm layers to contract and at the same time to 

 increase in thickness. The protoplasm consequently 

 undergoes a change in its form which necessitates a re- 

 arrangemevt of what arc knoivn as the plasmatic mole- 

 cules, that is the small particles within the protoplasm. 

 An inner tearing or cleaving takes place in the proto- 

 plasm both in freezing when the volume of the cell be- 

 comes decreased as well as in the thawing when the cell 

 again becomes di-stended. It is clearly this re-arrange- 

 ment of the plasmatic molecules which causes a cleaving 

 of its inner structure so that if dies. 



Some plants are very susceptil)le to such action. Others 

 again can withstand this to a certain degree if it only 

 takes place sufficiently slow in both directions. Frost 

 hardiness, however, is very different with one and the 

 same plant during different seasons and under different 

 conditions. The leaves of autumn-sown crops can with- 

 stand a temperature of 15 per cent C. (5 per cent F. 

 above zero) and even more for some time without being 

 injured. The newly formed leaves which appear in May 

 however may be damaged by a very slight frost. Trees 

 and bushes are more susceptible to fro.st and alternating 

 temperatures if they have begun to grow in the spring. 

 It is a peculiarity for protoplasm to be more susceptible 

 to the above mentioned re-arrangement of plasmatic 

 molecules when the temperature is comparatively high. 

 But if the' temperature becomes low, as in the autumn, 

 the protoplasm undergoes such a change that it is much 

 more hardy during the winter than during the summer." 



The varying ability of the protoplasm to withstand 

 this re-ai-rangement of its molecules is demonstrated by 

 the author in a simple experiment in which he takes a 

 living part of a plant and places it in water containing 

 a .solution of cane sugar, wiiich, because of its greater 

 concentration, is unable to penetrate the protoplasm. 

 The greater concentration of this li(iuid causes the water 

 within the cells, to be drawn out the same as in freez- 

 ing and consequently causes the protoplasm 'layers' to 

 be drawn together and thus to become thickened. A 

 microscopic examination of superficial cell layers which 

 have been sumersed in sugar solutions of different 



