484 SUMMARY OF CURRENT RESEARCHES RELATING TO 



he could make the dark-green colour of the lower side give way to the 

 normal colour, while the untouched parts remained infiltrated. This 

 observation prompted some experiments as to whether the infiltration 

 always disappeared with such remarkable rapidity on the thawing of 

 frozen leaves. Accordingly, infiltrated leaves of various plants were 

 plucked, and brought directly into an unheated room, the tempera- 

 ture of which was above zero, and the time in which the dark-green 

 colour was replaced by the normal was observed. In many cases, 

 this took place almost instantaneously, in others the time was longer, 

 but always a few minutes sufiiced for all traces of infiltration to dis- 

 appear, without the plant ever suffering through the rapidity of the 

 thawing. This phenomenon was general. 



A further question was, whether in this thawing, when the infil- 

 tration suddenly disappears, a rarefaction of air takes place in the 

 intercellular spaces, while the liquid returns into the cells. An 

 affirmative answer would prove that the leaves in freezing underwent 

 a diminution of volume, since the cells let out a part of their sap 

 into the intercellular spaces, and this forced out the air. To ascer- 

 tain the truth, Moll let the frozen leaves thaw under water, and 

 found that, after thawing, they were, indeed, more or less strongly 

 injected, and so that they had taken up water, becoming more or less 

 darkly coloured in consequence. He inferred that the frozen leaves, 

 on thawing, experience rarefaction of air in the intercellular spaces, 

 and therefore that the freezing is accompanied by a decrease of 

 volume of the leaves. 



The visible hanging down of the frozen leaves in many plants 

 formed the subject of a special inquiry, in which he made exact 

 measurements of the direction of the leaves during the process of 

 thawing. With a very simple apparatus, which served to measure 

 the fixation of the leaf on the petiole, and the direction of the leaf- 

 point, Moll satisfied himself that evergreen plants always change 

 their dii-ection in freezing, even where this change is not visible 

 without special means of observation. The time taken by the leaves 

 in thawing to reach their highest position varied in different plants 

 between seven and thirty minutes. On an average it was about 

 eighteen minutes. 



As to the rapidity of motion of the leaves, the experiments proved 

 that this at first increases, reaches a maximum, then decreases. The 

 increase, however, lasts but a short time, and the decrease is much 

 longer. Hence the maximum lies not in the middle, but more 

 towards the beginning, and it lasts only a very short time. 



Finally, Moll sought to ascertain whether the change of direction 

 of the leaves merely depended on their becoming more lax, or whether 

 other factors operated. In the former case leaves allowed to freeze 

 in an inverted position must equally sink down, and in thawing, they 

 must become erect, but thereby perform a movement which, under 

 normal conditions, would be a sinking. Experiment showed that the 

 laxity of the leaves, whose cells have given up water in freezing, is, 

 if not the only, yet the principal, cause of the down-hanging of frozen 

 leaves. 



