Page:An introduction to physiological and systematical botany (1st edition).djvu/119

 Rh been attributed to their being budded on a Pomegranate stock, of which I have never been able to obtain the smallest confirmation.

Heat can scarcely be denominated a secretion, and yet is undoubtedly a production, of the vegetable as well as animal body, though in a much lower degree in the former than the latter. The heat of plants is evinced by the more speedy melting of snow when in contact with their leaves or stems, compared with what is lodged upon dead substances, provided the preceding frost has been sufficiently permanent to cool those substances thoroughly. Mr. Hunter appears to have detected this heat by a thermometer applied in frosty weather to the internal parts of vegetables newly opened. It is evident that a certain appropriate portion of heat is a necessary stimulus to the constitution of every plant, without which its living principle is destroyed. Most tropical plants are as effectually killed by a freezing degree of cold, as by a boiling heat, and have nearly the same appearance, which is exemplified every autumn in the Garden Nasturtium, Tropæolum