533 



The mechanical action of frost is shown here in the form typical for 

 the majority of our plants since, on the upper side of the leaf, the collen- 

 chyma tissue aboxe the vascular bundle of the large vein is raised up from 

 the parenchyma, thereb}- forming an opening (/'). On the under side of 

 the leaf, the spongy parenchyma has been freed from the bark part of the 

 vein on the scarps of the very prominent body of the vein so that cavities 

 (h), containing air, are produced on both sides of the rib. The formation 

 of the caxities is explamed by the fact that the youthful, still hyponastic 

 leaf, the edges of which are up-curled, from the action of frost, C(jntracts 

 at both sides of the mid-rib from above downward, as well as tangentially. 

 If the up-curled, trough- like leaf contracts, the curling must become greater, 

 i. e., the distension of the under side becomes stronger. This manifests 

 itself in a pulling toward the raised edges (see the direction of the arrow in 



Fig-. 107. Cro.cis- sect ion through a t'lost boil m an apple leaf. 



the illustration). The tension is the greatest at the scarp of the vein and 

 can, at times, lead to a splitting of the epidermis (e'). 



If thawing now takes place, the result of the action of the frost is the 

 overlengthening of the tissue ^^-hich has been strained, for the tissues are 

 indeed distensible but not completely elastic. They do not regain their 

 former size and arrangement. The lower epidermis, which has been most 

 strained, elongates and no longer exercises on the spongy parenchyma, 

 lying beneath it, the previous amount of pressure. The pressure in the 

 epidermis is broken and the spongy parenchyma responds at once, elongat- 

 ing into pouches. If, at the time of the greatest tension, the epidermis is 

 torn apart, the over-elongated edges of the tear {e') form a crater-like 

 opening toward which grow out the rows (/) of the spongy parenchyma 

 which develop) into threads. 



