Every Girton girl (vice Macaulay’s schoolboy, retired from overwork) — every Girton girl knows that a well-conducted British oak “spreads its roots as far and wide through the soil beneath as it rears its boughs above toward the air of heaven.” Every Girton girl is probably also of opinion that the British oak does this mainly or solely in order to fix itself by firm anchors in the soil — to withstand the battling winds and the constant pull of hostile gravitation. But what every Girton girl does not, perhaps, quite so confidently know is this — that, on the whole, the tips of the roots and the tips of the branches correspond roughly in situation with one another, so that if you were to unearth and expose the entire tree you would find it composed of two tolerably similar domes or hemispheres — one erect and aërial, and one inverted and earth-bound, each occupying approximately equal areas, and each circumscribed by fairly equal circles.
Why should this be so? It is clear enough, of course, that in order to fasten a big tree firmly in the ground, it must have numerous large and strong foundations. But wherefore this approximate equality in the areas occupied by roots and frondage? The answer is, because every large tree forms a sort of umbrella, a domed roof or catchment basin for the rain that falls upon it; and it has always its own peculiar and admirably adapted arrangement for conducting all the water it intercepts to certain special spots or drinking-places in the ground, where it sets the roots, and especially the rootlets, or absorbent terminals, intended to soak that water up and convey it to the branches. If you stand under an oak-tree during a summer shower — a mode of passive scientific observation for which nature has afforded quite ample opportunities during the last few weeks — you will notice at once that the round mass of its foliage acts exactly like a huge umbrella, and conducts all the rain that falls upon its surface outward and downward towards the circumference of the circle. The drops that alight upon the central and tallest part of the tree are shed by the veined and channelled leaves till they fall off the tips on to the layer immediately below and outside them; this layer again conveys them to the next in order, and so on, till at last a little gathering stream drips from the ends of the lowest and longest outward-pointing boughs on to the soil beneath them. The ground in the centre remains perfectly dry, while a circle at the circumference is hollowed into a sort of irregular trench, or rude round of tiny pits, by the continuous dripping of the collected gutters.
Now, of course, the plant wants to utilize to the utmost all the rain it thus intercepts. It would be quite too silly of it to produce rootlets and absorbent terminals in the dry central space covered by the dense umbrella of foliage. But all around the circumference, and especially at the spots just under the runnels, where the water drops from the ends of the boughs, exactly as it drops from the rib-points of the silk-and-steel umbrella, the tree develops numerous minute rootlets, which suck up the rain as fast as it falls, and convey it by fixed pipes to the leaves and growing-points. Every tree and every large herb is thus a regular and well-organized catchment-basin, with its own mains and services; and it utilizes its water-supply by a cunningly adapted system of sucking rootlets, all placed at the exact spots where they will most surely absorb the amount of water that in each case runs down to them. So much is this true that in transplanting trees foresters and nurserymen know well you must lop the roots and the branches so as to cover equal superficial areas, or else the water will not fall on the parts best adapted to receive it; for, just as the lopped branches put forth new leaves and twigs at the point of section, so do the lopped roots put forth new rootlets and absorbent hairs at the place where they are now most urgently needed.
Not every kind of plant, however, manages its water-supply on the self-same system. There are dodges and devices. For herbs with leaves that spring from the rootstock alone, for example, without any visible above-ground stem, two main plans have been very widely adopted. One plan is that invented by plants like rhubarb, which have channelled leaves with grooved leaf-stalks, conducting all the water that falls upon their surface centrally towards the root. This is the centripetal type. Such plants resemble rather a funnel than an umbrella. They have always a straight tap-root, like a carrot; and this tap-root gives off numerous short rootlets on every side, which absorb all the water as it trickles down along the tapering surface of the inverted cone. The other plan — the centrifugal type — is adopted by certain plants with heart-shaped or arrow-shaped leaves, which have round leaf-stalks. In these cases the individual leaves point outward and downward, and the water drips from them not inward towards the centre, but outward towards the circumference. Their principle is rather umbrella-like than funnel-like. To meet this catchment system they have no long and descending tap-root, but just a short knobby root, which gives off long fibres radially in every direction; and these fibres terminate in knots or groups of absorbent rootlets exactly beneath the points where each leaf drips — the knobs or tags of the umbrella, to carry out our convenient metaphor. Examination of other and more complex plants reveals always the action of the same general law; each species has a peculiar catchment system of its own, more or less complicated, by means of which, directly or indirectly, all the water that falls upon its foliage is finally conducted to certain specified spots or drinking-places; and at those specified spots the plant provides beforehand an elaborate system of absorbent organs, exactly sufficient to suck up and utilize the average amount of water it expects to obtain and store at each of them. If London were a plant, now —— But hush! I am silent. A gathering frown on the reader’s brow warns me in time to steer clear of such human and political analogies.