FIRST ARTICLE.
It cannot have escaped the observation of the most inattentive, the tendencies which roots have generally to descend into the ground, and which stems have as commonly to grow upwards towards the sky; yet the very commonness of these things may have prevented their obtaining the attention that they merit; for it must be acknowledged, that to a mind directed to them they appear, however frequent their occurrence, not the less difficult to explain. It is sufficiently hard to comprehend why roots and stems should grow in different directions, the one downwards, and the other upwards; but when we add to these the constant manner in which the darker surface of a leaf is turned upwards, and the part of a flower painted with the most gorgeous colours is directed always towards the light, the subject becomes more interesting, and the more vexatious ought to be our ignorance: and, then, there are phenomena, produced by unusual circumstances, calculated to puzzle us still further, and increase our bewilderment. Such are the manner in which a geranium, growing at a window, bends its stems and leaves towards the glass; the manner in which a potato plant, growing in a cellar into which the light is admitted by a single chink, will acquire a most unusual height, and follow a most devious and uncommon track to reach that ray of which it appears enamoured; and the mode in which a root will descend, along the face of a bare rock, an extraordinary distance, in order to arrive at some spring or stream. These are objects well worthy of contemplation. A remarkable example of one of the facts just alluded to occurred many years ago in the tower of an old cathedral in England: a potato plant grew to the height of between thirty and forty feet, to get at the glimmering light of a partially closed window.
The final causes of many of these facts are easy to comprehend: the reason why a root grows down into the earth, is for the purpose of obtaining that sustenance which is necessary for the growth of the plant of which it is a part; and stems grow upwards, and towards the light, because the influence of this element is necessary for the elaboration of the sap; as a result of which process, stems grow in thickness, roots in length, flowers are developed, and the proper juices of vegetables become formed. We are likewise not without the means of explaining the proximate cause of one of these phenomena, for we have shown in our articles on Vegetable Sap that it is by the ascending sap that stems grow in length, and that, when light is excluded, no other sap can be formed; this causes the ascending sap to accumulate under such circumstances, and, consequently, in the dark, stems may be expected to acquire an enormous and very disproportionate length: thus we are enabled to understand why the potato, in the instance mentioned, should grow to so great a height. But admitting this explanation, how much seems incomprehensible in these common and too frequently neglected phenomena! We shall endeavour, in this and the following articles, to explain the manner in which these curious things occur.
One might imagine that the reason why roots grow into the earth, and stems grow out of it, is on account of the former being attracted, and the latter repelled, by the materials of which that earth is composed; or, on the other hand, by the stems being attracted, and roots repelled, by atmospheric air. But such cannot be the case; for if seeds be made to germinate in the lower stratum of earth placed in a box furnished with holes in the bottom, the roots will descend into the air through those holes, while the stems will ascend into the earth. In a similar manner, it might and has been thought that roots are attracted, and stems repelled, by the moisture of the earth; but a seed made to germinate between two moist sponges will protrude its root downwards, and its stem upwards, without reference to the liquid in its vicinity. This explanation is therefore equally inadmissible. There are some who explain these, as well as all other things occurring in living beings, by the mysterious principle of life; but we only admit the existence of this principle, because there are some phenomena incapable of being accounted for by the ordinary laws that rule the universe, and that are common to all matter; and it is therefore unphilosophical to ascribe any effects to its operation, until they are found to be inexplicable by those ordinary laws. But we shall find that the facts in question do not in a great measure belong to these exceptions.
The particular directions of stems and roots are produced by a combination of causes: if an onion plant, exposed to daylight, be laid horizontally on the ground, the extremities of the stem and roots will in the course of a few hours turn themselves in their natural directions, the one upwards, and the other downwards; if a similar plant be placed in a dark cellar, to which no light has access, the same things will take place; but that which happens in a few hours, in the one instance, will require as many days in the other; and thus we learn that in the production of these effects two causes operate: first, the light; and, secondly, some other principle distinct from light. It will occur to the reader that the absorption of water from the earth, by the most depending part of the plant, and its evaporation above, might, by swelling the lower portion and contracting the upper, produce the upward curving of the stem; to obviate this objection, the plant was placed in water, where no evaporation could occur, and absorption must take place equally over the whole surface; and still it was found that the same things happened.
Light, therefore, is most powerfully influential in producing the particular directions of the parts of plants; but there is another principle, distinct from light, which acts in effecting the same phenomena in a minor degree, but not the less absolutely and even more generally. Let our readers bear in mind the existence of this principle, which will form the subject of a future article. For the present, we will examine the manner in which light operates in promoting the directions of stems and roots.
We have before hinted that the tendency of the organs of vegetables towards the light, bears a direct relation to the depth and brilliancy of their colours; roots which are usually destitute of colouring matter grow away from the light; the upper surfaces of leaves are always the most deeply coloured; and in those erect leaves which are equally exposed to light, both surface are similarly coloured; if the outer surface of a flower be richly tinted, it is pendent; in erect flowers, on the contrary, the internal surface is always the most brilliantly painted; and in some cases the direction of the flower and fruit is different, connected with similar conditions. But in all these instances we have reason to believe that the organ is not directed towards the light, because it is highly coloured; but that it is highly coloured, because it is presented to the light. In plants growing in the dark, all the organs are colourless; it is only when exposed to the light that they acquire their various hues. Even the extremities of the roots have been found in a singular experiment of Dutrochet’s to acquire a green colour by exposure to the influence of light.
Is this tendency of the coloured parts of plants to turn towards the light, due to an attraction exerted by this agent, or is it produced by a peculiarity of growth determined through its influence? A curious experiment has settled this question: A leaf, attached by its footstalk to a pivot, was so arranged that it could freely turn in every direction: under these circumstances, its under surface was exposed to light. If an attraction existed between the most deeply coloured portion and the light, the leaf might be expected to revolve on its pivot, in obedience to this attraction: but instead, the footstalk took on a spiral or corkscrew growth, by means of which the upper portion became in time presented to the light. Now, this experiment sufficiently showed that the manner in which light acts, is by its influence over vegetable growth.
But what is the influence of light over vegetable growth? We have already answered this question in our articles on the Sap: we have found that when light is present, the sap becomes elaborated in the green parts of plants; and the use of this elaborated sap is, by developing vegetable fibre, to increase the thickness of stems, and the length of roots. While the ascending sap, by forming vegetable flesh, lengthens the stems, and makes the root thick, the directions of the different parts of plants, by the agency of light, must be in obedience to these functions.
We are now in a condition to comprehend the cause of some phenomena. A geranium (Pelargonium) stem, placed at a window, curves towards the light: this takes place, because the portion of stem nearest the window elaborates most sap: consequently, in this portion most vegetable fibre is formed. The portion away from the light, on the contrary, has most ascending sap, which forms fleshy tissue, and lengthens the stem; the half of the stem remote from the light is therefore longer, that next the window is shorter; the former is fleshy and elastic, the latter is rigid and fibrous. Need we be surprised, then, that the short, rigid, and fibrous portion should draw down the long, fleshy, and elastic part, and curve it towards the light?—it is but the bending of a bow, by the agency of its string.