304 Observations upon the effects of Frost, 



deeper green, and being of nearly the same depth of colour 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 Sinensis 

 in its natural state is examined, by tearing off the parenchyma from 

 the epidermis with violence, it will be found that the sphincter of its 

 stomates, the cells of the epidermis, and the chambers 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 epi- 

 dermis have collapsed, and touch each other, and all the cavernous 

 parenchyma below the epidermis is transparent, as if filled with fluid. 

 Whither the air is conveyed is not apparent ; but as the stomates 

 have evidently lost their excitability, 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, con- 

 tains 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 ex- 

 pelled 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 affected by frost was shown by their state in a thawed branch of 

 EuphorbiaTirucalli, when they were found in a collapsed state, empty 

 of both air and fluid, with their sides shrivelled, and with the fibre 

 itself, which forms the rings, also wrinkled transversely. Facts of an 

 analogous kind were remarked by me in Erica sulphurea. The 

 minute long-haired leaves of this species are in their natural 



