Effects of Frost on Plants, 25 



thousand pieces in the Horticultural Society's Garden, and a few 

 cases are cited in which the bark and trunk of trees was rent. 



" The expulsion of air from aeriferous organs, and the introduc- 

 tion of it into parts not intended to contain it, is a striking phenom- 

 enon. Every one must have remarked that when a leaf has been 

 frozen to death, it changes color as soon as thawed, acquiring a 

 deeper green, and being nearly of the same depth of color on both 

 sides ; the same appearance is produced by placing a leaf under the 

 exhausted receiver of an air-pump, and in both cases is owing to the 

 abstraction of air from the myriads of little air-chambers contained 

 in the substance of this organ. If the leaf of Hibiscus Rosa-Si- 

 nensis, in its natural state is examined, by tearing off the paren- 

 chyma from the epidermis with violence, it will be found that the 

 sphincter of its stomates, the cells of the epidermis, and the cham- 

 bers immediately below the latter, are all distended with air ; but 

 in the frozen leaf of this plant the air has entirely disappeared, the 

 sphincter of the stomates is empty ; the upper and under sides of 

 the cells of the epidermis have collapsed and touch each other, 

 and all the cavernous parenchyma below the epidermis is transpa- 

 rent, as if filled with fluid. Whither the air is conveyed is not 

 apparent ; but as the stomates have evidently lost their excitabil- 

 ity, and are in many cases open, it may be supposed that a part of 

 the air at least has been expelled from the leaf; and as the pith 

 of this plant in its natural state, contains very little air, and in the 

 frozen state is found to be distended with air, it is also probable 

 that a part of the gaseous matter expelled from the leaf when 

 frozen is driven through the petiole into the pith. In the petiole 

 of this plant are numerous annular and reticulated vessels, which 

 under ordinary circumstances, are filled with air, but after freezing 

 are found filled with fluid ; is it not possible that their functions 

 may have been disturbed by the violent forcing of air through 

 them into the pith, and that when that action ceased, they were 

 incapable of recovering from the overstrain, and filled with fluid 

 filtering through their sides ? That annular ducts are in some 

 way aff'ected by frost, was shown by their state in a thawed 

 branch of Euphorbia Tirucalli, when they were found in a collaps- 

 ed state, empty of both air and fluid, with their sides shrivelled, 

 and with the fibre itself, which forms the rings, also wrinkled trans- 

 versely. The minute long-haired leaves of Erica sulphurea are 

 in their natural state firm, bright green, with a rigid petiole, and 



Vol. XXXIX, No. 1. — April-June, 1840. 4 



