FREEZING AND BUKNING. 545 



stages of development are affected differently by the lowering of their temperature 

 below the freezing point. For the majority of plants the rule holds that death in 

 consequence of frost occurs the sooner, the younger and richer the tissues in question 

 are in water. The foliage of beeches, hornbeams, and deciduous oaks, which is not 

 killed in the autumn, even after repeated frosts, withers, shrivels, and dries up 

 when young if the temperature sinks below zero only for a single night in spring. 

 Even many Alpine plants which bear very low temperatures without injury, when 

 completely developed, may suffer harm if they are surprised by a frost at the period 

 of most active growth. When on one occasion at the end of June the temperature 

 sank to —6° on the mountains near Innsbruck, already free from snow, at an alti- 

 tude of 2000 metres, the young foliage-leaves of the Rhododendron {Rhododendron 

 hirsutum), which had just sprouted, and were not yet fully grown, were destroyed 

 on all the plants. They became brown and dried up, while the old fully-formed 

 green leaves, remaining on the same plants from the previous year, underwent no 

 alteration from the frost. 



Such phenomena can only be explained by the assumption that in young, un- 

 developed organs much water is present, and is not under the control of the living 

 protoplasm. As such water we may consider that which is conducted from the 

 roots to the green tissue, to be there liberated in the form of vapour; that which 

 mounts through the vascular bundles of the stem, streams through the veins of the 

 leaves, under certain circumstances is even forced into the intercellular spaces, and 

 passes out by the water-pores in the form of drops. This water is not retained by 

 molecular forces, nor protected against freezing, but turns into ice even at a tem- 

 perature of — 1°. Since it is abundantly present in the young tissue when freezing 

 takes place, extensive ruptures and mechanical injuries to the water-conducting 

 tubes and rows of cells are unavoidable. But if the conduction of the crude food- 

 sap is interrupted in a young plant-organ during its growth, transpiration in it can 

 no longer occur properly, and the transpiring cells become withered and dried up 

 even although their protoplasm should have suffered no direct harm from the frost. 



Naturally connected with this discussion is the question whether a plant can 

 freeze at a degree of temperature above that of the freezing-point of water. By 

 the majority of gardeners this question would be answered in the affirmative, and 

 their reply would be founded upon the fact that tropical Acanthacese, variegated- 

 leaved Coleus, basils, melons, tobacco-plants, &c., become withered, dry up and die 

 if they are exposed for only a single night to a temperature of -J- 2°. In spite 

 of the great outward resemblance between this phenomenon and freezing, it must 

 nevertheless not be called freezing, for the most distinctive processes of the freezing 

 of living protoplasm, viz. the excretion of water from the cell-body, the hardening 

 of this water into ice, and its inability to return to the cell-body, do not occur in 

 plants which are destroyed under the influence of temperatures above zero. It has 

 been clearly shown that this so-called freezing of plants at temperatures above zero 

 is really a drying-up in consequence of the disproportion between the transpiration 

 from the leaves and the absorption of water by the roots. In consequence of the 



VOL. I. 35 



