« PreviousContinue »
There, fed by food they love, to rankest size,
ON THE NATURAL CONDITIONS OF PLANTS.
To enter into any lengthened detail on the all-important subject of the natural conditions of plants would occupy far too much space; yet to pass it by without special notice, in any work treating of their cultivation, would be impossible.
Without a knowledge of the laws which regulate their growth, all our attempts must be empirical and more or less abortive. If we examine the vegetation on the surface of the globe, we shall find that the circumstances under which plants exist and flourish vary in an endless degree, and that they are all influenced by the atmosphere, heat, light, moisture, varieties of soil and periods of rest.
The purity of the atmosphere most sensibly affects the growth of plants, as evinced in the difference between those which grow in London and other large towns, or within the reach of manufactories evolving noxious gases, and those which grow in the country; but of this more hereafter.
The heat to which plants are subjected varies from 32° to 170°, or 180°. Thus, in some parts of Mexico, the
heat is so intense and the soil and atmosphere so dry, that no vegetation is found at certain seasons, save a species of Cactus, and were it not for this plant these tracts would form impassable barriers. Hardy states, in his Travels, that the sole subsistence of himself and party for four days consisted of the fruit of the Petaya, which, unlike most other luscious fruits, rather removes than creates thirst, while, at the same time, it satisfies to a certain degree the sensation of hunger. The providence of God is equally manifested in cold countries, as in Lapland, where the rein-deer moss furnishes the sole food, during winter, for the rein-deer, without which the inhabitants could not exist.
It is hardly possible to overrate the influence of light upon plants; but its intensity varies from almost total darkness to a light double that of our brightest summer's day. Upon light depend all the active properties, the color, &c. I am tempted to give an example from Mr. Ellis, of its effects in this last particular :-“In North America the operation of light in colouring the leaves of plants is sometimes exhibited on a great scale, and in a very striking manner. Over the vast forests of that country clouds sometimes spread, and continue for many days, so as almost entirely to intercept the rays of the sun. In one instance, just about the period of vernation, the sun had not shone for twenty days, during which time the leaves of the trees had reached nearly their full size, but were of a pale or whitish color. One forenoon the sun broke through in full brightness, and the color of the leaves changed so fast, that, by the middle of the afternoon, the whole forest for many miles in length exhibited its usual summer's dress."
The states of moisture vary as much as those of heat and light. The late Mr. Allan Cunningham often expressed to me his surprise at the extreme dryness of the atmosphere and soil in which grew many species of New Holland plants,
that in seasons when there was neither dew nor rain, he had dug several feet below their roots without finding a trace of water, and yet Banksias and Acacias would continue to live in this state for a considerable time. There are numerous other plants, independently of those which live in water, which cannot exist unless the atmosphere and soil are continually humid, such as Trichomanes speciosum, &c. &c. Plants are affected by soils — sometimes specifically, but more generally in consequence of various soils possessing different powers of imbibing and retaining moisture. All plants require rest, and obtain it in some countries by the rigour of winter; in others by the scorching and arid heat of summer. Some “ after short slumber wake to life again," while the sleep of others is unbroken for many months. This is the case with most alpine plants, and is necessary to their well-being. Messrs. Balfour and Babington, whilst recently exploring the lofty mountains of Harris, found the climate to be so modified by the vicinity of the Great Atlantic Ocean, that, notwithstanding their northern latitude (58°), many of the species inl ting the Highland districts of Scotland were wholly wanting, and that the few which they saw were confined to the coldest and most exposed spots. From the same cause many plants grow there which were not known to grow
in so northern a latitude in Britain. In Egypt the blue water-lily obtains rest in a curious way. Mr. Traill, the gardener of Ibrahim Pacha, informed me that this plant abounds in several of the canals at Alexandria,