Transactions of the London Horticiillural Society. 511 



have remarked that when a leaf has been frozen to death, it changes colour as 

 soon as thawed, acquiring a 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 vio- 

 lence, 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 epidermis have collapsed, and touch each other, and all the ca- 

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

 Whither the air is conveyed is not apparent ; but as the stomates have evi- 

 dently 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, 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 affected by frost was shown by their state in a thawed 

 branch of Euphorbia Tirucalli, 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 state firm, bright green, with a rigid petiole, 

 and upon being exposed to pressure in a com-pressorium, at first offer perceptible 

 resistance to its action, and afterwards, as the pressure increases, discharge, 

 chiefly through their petiole, a great quantity of air. But leaves of this 

 plant, which have been frozen by exposure to the temperature of 27° are very 

 different ; they are softer, dull olive green, with a flaccid petiole, and offer but 

 little resistance to pressure ; yet, although they give way freely, the quantity of 

 air which the compressorium expels is comparatively small, and readily driven 

 out. Moreover, the long hairs of this plant, which in the natural state are 

 occupied by fluid, were always found filled with air after freezing, and this 

 without pressure having been exercised upon them. 



" I am inclined to refer to this cause the well-known fact, of which many 

 cases occurred this winter, that the sudden exposure of frozen plants to 

 warmth will kill them ; though they may not suffer if warmed gradually. In 

 such cases, it may be supposed that the air, forced into parts not intended to 

 contain it, is expanded violently, and thus increases the disturbance already 

 produced by its expulsion from the proper air cavities ; while, on the other 

 hand, when the thaw is gradual, the air may retreat by degrees from its new 

 situation without producing additional derangement of the tissue. It is also 

 possible that leaves, from which their natural air has been expelled by the act 

 of freezing, may, from that circumstance, have their tissue too little protected 

 from the evaporating force of the solar rays, which we know produce a specific 

 stimulus of a powerful kind upon those organs. 



" These circumstances are, in themselves alone, sufficient to account for 

 death being produced in plants by frost ; and it is chiefly to such as these, 

 that Professor Morren has directed his attention. It however appears to me 

 that there are some other points of importance to which observers have not 

 applied themselves. 



" The green colouring matter of leaves, or chlorophyll, is certainly affected 



L L 4 



