156

NATURAL GROUND TEMPERATURES.

II. A TABLE OF THE ASCERTAINED, OR ESTIMATED AVERAGE GROUND TEMPERATURES IN DIFFERENT PARTS OF THE WORLD.

The temperature here estimated as that of the season of rest is to be understood as what roots are probably exposed to at a foot below the surface. Of course, upon the surface, the temperature would be lower.

CHAPTER II.

OF THE MOISTURE OF THE SOIL.-WATERING.

Ir has already been shown that water is one of the most important ingredients in the food of plants, partly from their having the power of decomposing it, and partly because it is the vehicle through which the soluble matters found in the earth are conveyed into the general system of vegetation. Its importance depends, however, essentially upon its quantity.

We know, on the one hand, that plants will not live in soil which, without being chemically dry, contains so little moisture as to appear dry; and, on the other hand, an excessive quantity of moisture is, in many cases, equally prejudicial. The great points to determine are, the amount which is most congenial to a given species under given circumstances, and the periods of growth when water should be applied or withheld.

When a plant is at rest, that is to say, in the winter of northern countries and the dry season of the tropics, but a small supply of water is required by the soil, because at that time the stems lose but little by perspiration, and consequently the roots demand but little food; nevertheless, some terrestrial moisture is required by plants with perennial stems, even in their season of rest, because it is necessary that their system should, at that time, be replenished with food against the renewal of active vegetation. Hence, when trees are taken out of the earth in autumn, and allowed to remain exposed to a dry air all the winter, they either perish, or are greatly enfeebled. If, on the other hand, the soil in which they stand is filled with moisture, their system is distended with aqueous matter at a time when it cannot be decomposed or thrown off, and the plant either

160

EFFECTS OF EXCESSIVE WINTER-WET.

loses its roots by rotting, or becomes unnaturally susceptible of the influence of cold in rigorous climates, or is driven prematurely into growth, when its new parts perish from the unfavourable state of the air in which they are then developed. The most suitable condition of the soil, at the period of vegetable rest, seems to be that in which no more aqueous matter is contained than results from the capillary attraction of the earthy particles.

During the season of 1852 and 1853, in which rain fell, with little intermission, from November till March, and inundated permanently gardens in low situations, Rhododendrons, although fond of moisture, perished to a great extent. During winter they seemed to be healthy, but when spring arrived their leaves became dull, changed to brown, and withered, and the buds refused to push; or, when attempts were made by plants to renew their vegetation, their growth was feeble, and most of them died in the course of the following autumn or winter.

Nevertheless, there are exceptions to this, in the case of aquatic and marsh plants, whose peculiar constitution enables them to bear with impunity, during their winter, an immersion in water; and in that of many kinds of bulbs, which, during their season of rest, are exposed to excessive heat and dryness. The latter plants are, however, constructed in a peculiar manner; their roots are annual, and perish at the same time as the leaves, when all the absorbent organs being lost, the bulb cannot be supposed to require any supply of moisture, inasmuch as it possesses no means of taking it up, even if it existed in the soil.

The conditions under which true aquatics exist have been so well explained by a philosophical writer, in the Gardener's Chronicle, 1852, p. 19, that I quote his statement without curtailment, although his remarks, in part, refer to other questions besides the moisture of the medium in which roots are placed :

Plants growing entirely under water are to some extent protected from those great and sudden changes of temperature to which ordinary land plants are frequently exposed; at the same time, however, water plants are very often injured by cold, and it not unfrequently happens, that on a cold winter's night plants in a pond will be greatly injured, or even killed, whilst those in a neighbouring pond will remain quite uninjured. In order to understand the precise cause of this phenomenon, we must examine the conditions under which plants grow, and the peculiar sources of injury to which they are consequently exposed.