The amount of cold that plants are capable of resisting is a question of much interest to fruit cultivators; and it becomes the more interesting when we reflect that this power is dependent upon circumstances, which are, to a certain extent, under the control of the cultivator. It is no uncommon circumstance to find a plant at one time killed with a less degree of cold than it had previously endured without the slightest injury; even the hardiest of trees will be injured by slight frost if they are subjected to it when their power of repelling cold is at its minimum, and this is a frequent, secret and unseen cause of disease and death.

While it cannot be shown that frost is actually beneficial to plants, we know that its injurious effects are not always immediately visible, and that it is a growing and well-based opinion, founded upon close observation, that many of the diseases of trees are the result of repeated injuries from the frosts,.and extreme changes of temperature during winter.

Notwithstanding the importance of this question, it has not received that attention from practical men that it deserves, and even the contributions of science, although highly valuable, have not been of a nature to render much practical aid in this department of horticulture.

The theory propounded by De Candolle, may be here quoted, namely, that, as a general rule, the power of plants to resist extremes of temperature is: First, in the inverse ratio of the quantity of water which they contain. Secondly, in proportion to the viscidity of their fluids. Thirdly, in the inverse ratio of the rapidity with which their fluids circulate. Fourthly, in proportion to the size of the cells, so is the liability of plants to freeze. Fifthly, the power of plants to resist the extremes of temperature, is in exact proportion to the amount of confined air which the structure of the plants themselves enables them to contain!, These and other principles are promulgated; ana apart from practical observation are sufficient to form the groundwork for theory. There is not much, however, in the above calculated to be of material aid to the cultivator. He cannot ascertain the dimensions of the cells, any more than he can measure the quantity or decide upon the quality of the fluids with which they are filled. The wood of the orange tree is to all appearance as close and hard as an oak, yet the former will not stand our winters.

The willow and the fig have the softest and the lightest of wood, the one is hardy and the other is not.

Although physiologists have not been able to give us broad and well-defined distinctions, or any definite explanation why one plant is hardier than another, except that its constitution is adapted to its natural climate; we know that all plants are rendered more capable of resisting extremes when their wood is properly matured, or ripened.

The ripening process consists in the slow and complete removal of watery matter, and the conversion of fluid organizable matter into the more solid substances which are necessary to form the woody secretions of the plant. This is, so far, in accordance with the theory of De Candolle, since the riper the wood, the dryer is its tissue, and the more solid its secretions.

The effects of thorough ripening of the wood is not only seen in the power it confers of resisting cold; a more important result is that it provides an abundance of the secretions necessary to sustain the growth of the following spring, and produce the flower buds upon which the hopes of the florist as well as the orchardist are founded; it is well known that flowers will not be produced upon the apple, pear, or strawberry any more than upon the camellia or rose, unless the elements of growth have been sufficiently abundant, and presented in due relative proportions to perfect previous growth.

Referring to these well-known facts, we see how far it is in our power to assist nature in supplying the requisites for perfect maturation of growth. The fruit-grower will be careful that his trees are not planted in wet, or highly enriched soil, that would tend to prolonged growth in the fall - that his strawberry plants are not overgrown by weeds after the crop is gathered, but are carefully cleaned and thinned - that his raspberry plants have been divested of all old wood as soon as the crop was removed, and the young growth thinned to proper distances, and disposed to the full enjoyment of light and air.

The florist will learn how his tender roses may be rendered more hardy, and the absurdity of attempting to force flowers on a camellia in spring, that had not formed the flower-buds during the previous summer. . And the amateur may also experiment, and with great hopes of success, upon his Cryptomerias, Deodars and Washingtonias, that are growing excessively luxuriant; by an early check to growth, either by covering the ground to prevent the ingress of water to their roots, or, sever a few of the main roots sometime previous to the natural completion of growth, so that the shoots will become dry and hard, the buds plump and ripe; and the plant, before being overtaken by frost, be in a comparatively dormant state, and the shoots and bark, instead of being unripe and full of sap, be mature, hard, firm, and quite prepared for sudden and extreme changes of temperature. A few years of such treatment would probably induce permanent hardiuess, as many plants perfectly hardy when old, are easily killed down when young.

This we consider the most important, and indeed the only true safeguard against injury from frost.