[55] Our experiments have not yet been carried so far, as to determine if, by any arrangement of drying or exposure, they may be seasoned to sustain intense frost, which may affect them differently from moderate frost, either by causing complete congelation of all their structure (moderate freezing appearing only to congeal their fluids, but not entirely the containing vessel, at least only partly congealing the mass), or by killing the vital principle itself through nervous affection. The potatoes became green from the exposure to the light, and we rather think acquired greater hardihood of constitution, or greater vitality or excitability by the exposure, thence greater power to resist the cold, independent of the disposition they acquired by desiccation to endure it. [56] Is the rending of the stems of trees, during intense frost, internal only, and occasioned by the alburnum expanding more by congelation than the drier mature wood? or, is it external, and caused by the contractile effect of the dry air and cold on the alburnum rendering it insufficient to surround the mature wood, which, from dryness and want of living susceptibility, may not contract so much. [57] The fineness of vessel or fibre of the Siberian crab, may be induced by the arid warm air, the continued radiation of heat and light upon the portion above ground, and the coldness of the ground around the roots during the short summer in Siberia, where the air and surface of the ground is warm, and vegetation progressive, while the ground remains frozen at a small depth. Like all varieties of plants habituated to colder climate, the Siberian crab developes its leaves under less heat than varieties of the same kind which have been habituated to milder climate. [58] We have not taken Sir Henry in the literal sense. Timber is well known to decay sooner in a warm than in a cold country, _cæteris paribus_. [59] See Appendix F. [60] The preliminary sentence is very vaguely worded; we suppose, “increasing the annual circles,” means increasing them in thickness, not general contents of length multiplied by thickness. But even in the latter sense, we hold pruning tends generally to diminish the annual circles. [61] It is a theory of Mr Sheriff, Mungo’s Wells, that all plants have excrementitious deposit from the roots, the deposit from one kind affording a good manure to another kind. Thence the advantage of mixed grasses and legumes in pastures, and of the rotation of different kinds of crops. [62] Vegetable soil is sometimes buried deep under volcanic mud, sand, and ashes, or mixed with the subsoil by earthquakes. In some districts of South America, the country, from being fertile, has been recently reduced to sterility, by the vegetable mould being so much scattered through the subsoil by repeated upheavings and tossings about by earthquakes, as to be out of the reach of plants.