286 DISEASES OF TREES 



When the tissues of the leaves and cortex, and in fact when 

 any parenchymatous tissues are frosted, pure water is withdrawn 

 into the adjoining intercellular spaces, but the cells themselves 

 do not generally freeze. The result is that the cells lose their 

 turgidity, and at the same time begin to droop. This explains 

 the familiar phenomenon of lilies, hyacinths, &c., which have been 

 caught by late frost, being prostrated on the ground, until the 

 ice melts and the cells reabsorb the water into their interior 

 and again become turgid, when the plants resume an erect 

 position. 



Cells which contain a concentrated solution^part with water 

 only under the influence of very intense cold, and I have often 

 found that the cortex and bast of trees showed no signs of ice 

 when the wood was hard frozen. 



As a rule, when living plant-tissues that contain much water are 

 frosted and this applies especially to young leaves and shoots 

 that are affected by late frost large masses of ice are formed in 

 certain regions, and notably underneath the epidermis of leaves 

 and shoots, and in the medulla. The tissues, however, remain 

 entirely free of ice, merely shrinking in proportion to the 

 quantity of water that is lost. These masses of ice consist of 

 parallel prismatic crystals, which are arranged at right angles 

 to the tissues from which the water has been abstracted. The 

 cortical parenchyma of the shoot usually contains numerous 

 intercellular spaces, especially along the line that marks the 

 limits of the collenchymatous tissues of the outer cortex. 

 Owing to the formation of a sheet of ice in this region, a 

 separation of the cortical tissues may take place, which however 

 may occasion but little damage to the plant. I have noticed 

 after a late frost that the epidermis on the under side of the 

 leaves of the sycamore was pushed out into numerous vesicular 

 swellings, but it was only after the lapse of several weeks that 

 this forcible separation exercised any prejudicial influence on the 

 health of the leaves. 



On account of its numerous large intercellular spaces, it is 

 evident that the spongy parenchyma of the under part of the 

 leaf offers specially favourable conditions for the formation of ice. 

 In the case of the false acacia and other trees that are still green 

 when the first frost occurs in autumn, a sheet of ice forms in the 



