By Professor Lindley. 



305 



state firm, bright green, with a rigid petiole, and upon being ex- 

 posed to pressure in a compressorium, at first offer perceptible re- 

 sistance 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 expul- 

 sion 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, w hich 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 how- 

 ever 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 



