Roots may be produced of a proper description for planting by the adoption of such means as the above, and if care be taken they will suffer little from the operation of removal. Still they may be seriously injured from exposure to air, and more especially to drying winds, frost, and even to wet. To be covered in the soil is the natural con dition of the roots of most vegetable productions, and therefore endeavours should be made to place them in such a condition with the least possible delay ; or at all events, if circumstances render delay unavoidable, such means should be adopted as will preserve them in a state of moisture similar to that which they have in the soil. It is however necessary to observe, that when the tops are closely packed up and evaporation from them prevented, the roots should be kept rather dry than otherwise ; for under such circumstances, damp is found to be much more frequently destructive than a little dryness.
Though the preparatory steps to planting, as regards roots and their preservation, may be properly taken, yet, if the operation be not per formed at the proper season, success will not be complete. It is true that instances may be adduced of planting being done at the very opposite season to that which is here recommended as the beet ; but such cases are only examples of what may be done by extraordinary care in adopting artificial menus so as to imitate more natural circum stances. It is a well-known fact that plants are subject to a process of evaporation from all the parts which are exposed to the atmosphere, provided the latter is not in a state of saturation. The source from whence this evaporation is supplied is the moisture of the soil, and the roots are the media by which it is collected and transmitted to the stem, branches, and leaves. If the expenditure be greater than the supply of moisture, the plant will begin to suffer ; if the disparity be great and continued, a degree of desiccation will be occasioned sufficient to deprive the vegetable tissue of that moisture which constitutes the medium of intercommunication between the different cells ; organi sation is destroyed ; the chain which connected it with vitality is broken, and incapable of being re-united by any mechanical means.
A much greater amount of evaporation takes place from a given surface in some species than in others ; but the amount as regards plants of the same species, all other circumstances being the same, is in proportion to the extent of surface which the respective individuals possess. The leaves of a lime-tree may have a surface equal to thirty times that of the stem, branches, and twigs which produce them, and consequently the demand on the roots for the supply of evaporation will be thirty times greater when in leaf than when the branches are naked. Instances might be adduced in which this difference would
even be exceeded; but if, on the contrary, it were found considerably still there would be sufficient evidence of the impropriety of moving plants when in leaf; and it may be here remarked, that young leaves admit of the fluids being more rapidly evaporated than old ones.
The month of December is that in which the condition of the air with regard to moisture approaches nearest to that of complete saturation, and next in this respect is January, which is also the coldest. It might therefore be concluded that these months are the best for planting, more especially as they are the most opposite in character to June and July, which, from their being the hottest and "driest, are found to be the worst. But November is also moist ; and although the temperature of the season has not then reached its minimum, yet the foliage, accustomed to exercise its functions under a summer heat, can no longer perform them under a reduction of temperature, which, though not the lowest, is comparatively low as regards summer productions. The leaves in consequence lose their connection with the roots ; the earth still retains considerable warmth ; and although the absence of leaves in deciduous trees, and perhaps the inactivity of those of evergreens, may prevent the formation of wood, except in the most limited degree, yet the buds, whilst they are them selves increased in volume, maintain a corresponding action in the roots, sufficient in many instances to form fresh spongioles before winter, a process which is favoured by the soil being warmer than the atmosphere. Should circumstances occur to prevent the actual pro trusion of cellular substance in the form of spongioles, still an accumulation of it will be taking place, ready to burst forth in spring. On this account therefore, although December, January, and February are sufficiently moist, yet November, or as soon as the leaves have fallen, is the preferable season for planting. There may be some exceptions, as in the case of very wet soils, where the plants, if not firmly rooted, are liable to be thrown out during winter, owing to which spring planting would be more proper. But uudcr ordinary circumstances all deciduous trees will succeed best at the period above indicated. Such species as push forth their buds early ought certainly to be planted in autumn. Many of the coniferous tribe will succeed well If planted soon after they have made their summer's growth : the earth is then warm, and the plants make roots very promptly. Some species of this interesting order of trees require to be planted either before winter, so as to have sufficient time to strike root, or immediately before their buds begin to burst in the spring.