Comparative Physiology. 341 



the latex and forming the organism, must be productive of a 

 considerable amount of heat, which, though not perceptible in 

 any great degree to a thermometer on the outside, must be 

 of essential consequence where it is generated, and probably- 

 indispensable to the activity of life. Whether the heat in these 

 vital actions flows from the action of the vital principle itself, as 

 thought by some, or from the chemical action displayed, or 

 partly from both, it is undoubtedly present, and therefore 

 needed. Whatever states of the weather or soil produce cold must 

 be greatly detrimental to vegetation, and are undoubtedly the 

 causes of many diseases. The cold arrests the proper develoj^e- 

 ment of growth, and produces diseases in the leaves and young 

 shoots, which may be seen in many various forms ; and, if long 

 continued, and joined to other injurious circumstances, often 

 ends in producing a cancerous state of the system, and death. 

 That cold is sometimes in a great measure the cause of canker, 

 may be seen in the fact that trees subject to it in ordinary 

 situations are not so when trained to walls. When the tissue 

 is ripened, as in deciduous plants, before wintei', and to a certain 

 extent also in evergreens, plants will stand a great degree of 

 cold. Heat is much more easily conducted along than across 

 the woody fibre ; and the slow conducting power across, and the 

 comparatively dry condition of the tissues, prevent the danger 

 arising from severe cold in the winter : it is only in the polar 

 regions that it is sometimes so intense as to split the trunks of 

 deciduous trees. Evergreens, from the more fluid nature of the 

 tissues, and the greater degree of vital action, are more easily 

 hurt ; and hence, while many of the deciduous plants of America 

 will not ripen sufficiently here, from deficient heat in the sum- 

 mer, and perish during winter, our most common evergi^eens, 

 which in ordinary seasons are not hurt here, will not stand the 

 greater severity of an American winter. Seeds are seldom or 

 never hurt in the severest winter ; the smallest of our seeds will 

 lie on the surface of the ground without injury, as far as cold is 

 concerned, but are considerably and easily injured by the drying 

 power of air, or heat without moisture. Roots of deciduous 

 plants, well ripened, do not appear easily hurt by cold either, 

 as they stand often, when the frost is excessive and long con- 

 tinued, completely insulated in a mass of frozen earth, without 

 the least injury ; though they perish, if the roots are long exposed 

 to the drying power of air, much sooner than seeds. A certain 

 degree of moisture around the roots appears necessary to keep 

 the plant alive, and would therefore seem to argue some degree 

 of active vitality in the roots ; and perhaps heat and some small 

 portion of moisture are conducted upwards even in winter : it 

 is only in very mild winters, however, that any outward mani- 

 festations, by protrusion of spongioles, make their appearance. 

 The sources of the evolution of heat in organised beings have 



