An introduction to physiological and systematical botany/Chapter 12
OF THE ROOT, AND ITS DIFFERENT KINDS.
We begin the description of the completely formed vegetable by its Root, as being the basis of all the rest, as well as the first part produced from the seed. Its use in general is two-fold; to fix the plant to a commodious situation, and to derive nourishment for its support. This part is therefore commonly plunged deeply into the ground, having, as we have already shewn, a natural tendency to grow downwards. In some cases however, when plants grow on the stems or branches of others, as the Dodder or Cuscuta, several Ferns, and a portion of the Orchis tribe, the root is closely attached to the bark, from which it draws nourishment, by the under side only, the upper being bare.
The Root consists of two parts, Caudex, the body of the Root, and Radicula the fibre. The latter only is essential, being the part which imbibes nourishment.
Roots are either of annual, biennial or perennial duration. The first belong to plants which live only one year, or rather one summer, as Barley; the second to such as are produced one season, and, living through the ensuing winter, produce flowers and fruit the following summer, as Wheat; and the third to those which live and blossom through many succeeding seasons to an indefinite period, as trees, and many herbaceous plants. The term biennial is applied to any plant that is produced one year and flowers another, provided it flowers but once, whether that event takes place the second year, as usual, or whether, from unfavourable circumstances, it may happen to be deferred to any future time. This is often the case with the Lavatera arborea, Tree Mallow, and some other plants, especially when growing out of their natural soil or station. Linnæus justly observes that however hardy with respect to cold such plants may prove before they blossom, they perish at the first approach of the succeeding winter, nor can any artificial heat preserve them. This is, no doubt, to be attributed to the exhaustion of their vital energy by flowering. Several plants of hot climates, naturally perennial and even shrubby, become annual in our gardens, as the Tropæolum, Garden Nasturtium.
In the Turnip, and sometimes the Carrot, Parsnep, &c., the Caudex or body of the root is above-ground and bare, becoming as it were a stem. Linnæus indeed calls the stems of trees "roots above-ground;" but this seems paradoxical and scarcely correct. Perhaps it would be more accurate to say the caudex is a subterraneous stem; but we rather presume it has functions distinct from the stem, analogous, as has been hinted p. 55, to digestion, at least in those plants whose stems are annual though their roots are perennial.
The fibres of the root, particularly those extremities of them which imbibe nourishment from the earth, are in every case strictly annual. During the winter, or torpid season of the year, the powers of roots lie dormant, which season therefore is proper for their transplantation. After they have begun to throw out new fibres, it is more or less dangerous, or even fatal, to remove them. Very young annual plants, as they form new fibres with great facility, survive transplantation tolerably well, provided they receive abundant supplies of water by the leaves till the root has recovered itself.
Botanists distinguish several different kinds of roots, which are necessary to be known, not only for botanical purposes, but as being of great importance in agriculture and gardening. The generality of roots may be arranged under the following heads.
1. Radix fibrosa. A Fibrous Root. The most simple in its nature of all, consisting only of fibres, either branched or undivided, which convey nourishment directly to the basis of the stem or leaves. Many grasses, as Poa annua, Engl. Bot. t. 1141, and the greater part of annual herbs, have this kind of root. The radical fibres of grasses that grow in loose sand are remarkably downy, possibly for the purpose of fixing them more securely to so slippery a support, or to multiply the surface or points of absorption in so meagre a source of nutriment. The fibres of some parasitical plants already alluded to, chiefly of the beautiful genus Epidendrum, are peculiarly thick and fleshy, not only for the purpose of imbibing the more nourishment, but also to bind them so strongly to the branches of trees, as to defy the force of winds upon their large and rigid leaves.
2. Radix repens. A Creeping Root, as in Mint, Mentha. A kind of subterraneous stem, creeping and branching off horizontally, and throwing out fibres as it goes. This kind of root is extremely tenacious of life, for any portion of it will grow. Hence weeds furnished with it are among the most troublesome, as the different sorts of Couch-grass, Triticum repens, Engl. Bot. t. 909 Holcus mollis, t. 1170, &c.; while, on the other hand, many sea-side grasses, having such a root, prove of the most important service in binding down loose blowing sand, and so resisting the encroachments of the ocean. These are principally Carex arenaria, Engl. Bot. t. 928, Arundo arenaria, t. 520, and Elymus arenarius, t. 1672.
3. Radix fusiformis. A Spindle-shaped, or Tapering Root. Of this the Carrot, Parsnep and Radish are familiar examples. Such a root is formed, on the principle of a wedge, for penetrating perpendicularly into the ground. It is common in biennial plants, but not peculiar to them. The caudex, which is the spindle-shaped part, abounds with the proper secreted juices of the plant, and throws out numerous fibres or radicles, which are in fact the real roots, as they alone imbibe nourishment.
4. Radix præmorsa. An Abrupt Root, is naturally inclined to the last-mentioned form, but from some decay or interruption in its descending point, it becomes abrupt, or as it were bitten off. Scabiosa succisa, Devil's-bit Scabious, Engl. Bot. t. 878, Hedypnois hirta, t. 555, and some other Hawkweeds, have this kind of root, the old opinion concerning which cannot be better described than in Gerarde's Herbal, under the plant first named, p. 726.
"The great part of the root seemeth to be bitten away: old fantasticke charmers report, that the divel did bite it for envie, because it is an herbe that hath so many good vertues, and is so beneficial to mankinde."———The malice of the devil has unhappily been so successful that no virtues can now be found in the remainder of the root or herb.
5. Radix tuberosa. A Tuberous or Knobbed Root, is of many different kinds. The most genuine consists of fleshy knobs, various in form, connected by common stalks or fibres, as in the Potatoe, Solanum tuberosum, and Jerusalem Artichoke, Helianthus tuberosus Jacq. Hort. Vind. t. 161. These knobs are reservoirs of nourishment, moisture, and vital energy. Several of the Vetch or Pea kind are furnished with them on a smaller scale; see Vicia lathyroides, Engl. Bot. t. 30, and several species of Trifolium, either annuals, as glomeratum, t. 1063, or perennials, as fragiferum, t. 1050.—The knobs in these instances are only of annual duration; in the Paeonia, Paeony, t. 1513, and Spiræa Filipendula, Dropwort, t. 284, they are perennial.—In the Orchideæ of Europe they are mostly biennial. The root in many of the latter consists either of a pair of globular or oval bodies, as in Satyrium hircinum, Engl. Bot. t. 34, Ophrys aranifera, t. 65, and apifera, t. 383; or are palmate, that is, shaped somewhat like the human hand, as in Orchis maculata, t. 632. Of these globular or palmate knobs or bulbs one produces the herb and flowers of the present year, withering away towards autumn, and the other is reserved for the following season, while in the mean time a third is produced to succeed the latter. The knobs of Ophrys spiralis, t. 541, are formed three or four years before they flower, and their flowering appears to be occasionally deferred to a more distant period. The root of Satyrium albidum, t. 505, consists of three pairs of tapering knobs or bulbs, which flower in succession. On the contrary, Ophrys monorchis, t. 71, forms its new bulb so late that it is not perfected till the autumn immediately preceding its flowering, and the plant seems to have but one bulb. Ophrys Nidus avis, t. 48, has clusters of cylindrical knobs, which are formed, and also wither away, in parcels, each parcel being equivalent to one of the abovementioned bulbs.
Such of the Orchis tribe as have biennial bulbs are supposed to be very difficult of cultivation, but according to the experience of my excellent friend the late Mr. Crowe, in whose garden I have seen them many successive years, they are best removed when in full flower, the earth being cleared completely away from the roots, which are then to be replanted in their natural soil previously dried and sifted. Afterwards they must be well watered. The bulb for the following year has not at the flowering period begun to throw out its fibres, for after that happens it will not bear removal. Satyrium albidum having, as mentioned above, so many pairs of roots, the growth of some of which is always going on, has hitherto not been found to survive transplantation at all.
Iris tuberosa, Sm. Fl. Græc. Sibth. t. 41, has a root very analogous to these just described, but I. florentina and I. germanica, t. 39 and 40 of the same work, have more properly creeping roots, though so thick and fleshy in their substance, and so slow in their progress, that they are generally denominated tuberous.
6. Radix bulbosa. A Bulbous Root, properly so called, is either solid, as in Crocus, Ixia, Gladiolus, &c.; tunicate, tunicata, composed of concentric layers enveloping one another as in Allium, the Onion tribe; or scaly, consisting of fleshy scales connected only at their base, as in Lilium, the White or Orange Lily. The two latter kinds have the closest analogy with leaf-buds. They are reservoirs of the vital powers of the plant during the season when those powers are torpid or latent, and in order to perform the functions of roots, they first produce fibres, which are the actual roots. The strict affinity between bulbs and buds appears from the scaly buds formed on the stem of the Orange Lily, Lilium bulbiferum, which fall to the ground, and, throwing out fibres from their base, become bulbous roots. The same thing happens in Dentaria bulbifera, Engl. Bot. t. 309, and Saxifraga cernua, t. 664.
These two last-mentioned plants however have scaly roots, like the Toothwort, Lathræa Squamaria, t. 50, which seem bulbs lengthened out. Whether they would, in the torpid season of the year, bear removal, like bulbs, we have no information. If disturbed at other times they are immediately killed. Many plants with solid bulbs are provided by Nature to inhabit sandy countries, over the face of which, in the dry season succeeding their flowering they are scattered by the winds to a great distance, as happens to our own Poa bulbosa, Engl. Bot. t. 1071, as well as to numerous beautiful productions of the Cape of Good Hope.
7. Radix articulata, or granulata. A Jointed or Granulated Root agrees very much with those described in the last section. The Oxalis Acetosella, Wood Sorrel, Engl. Bot. t. 762, and Saxifraga granulata, White Saxifrage, t. 500, are instances of it. The former has most affinity with scaly bulbs, the latter with solid ones.
It is evident that fleshy roots, whether of a tuberous or bulbous nature, must, at all times, powerfully resist drought. We have already mentioned, p. 41, the acquisition of a bulb in Phleum pratense, Engl. Bot. t. 1076, whenever that grass is situated in a fluctuating soil, by which its vital powers are supported while the fibrous roots are deprived of their usual supplies. In this state it becomes the Phleum nodosum of authors; but on being removed to a thoroughly wet soil, it resumes the entirely fibrous root, and luxuriant growth, of Ph. pratense. I have also found Alopecurus geniculatus, t. 1250, (an aquatic grass, whose root is naturally fibrous and creeping,) growing with an ovate juicy bulb on the top of a dry wall. This variety has been taken for the true A. bulbosus, t. 1249, which has always bulbs even in its native marshes. We see the wisdom of this provision of Nature in the grasses above mentioned, nor may the cause be totally inexplicable. When a tree happens to grow from seed on a wall, it has been observed, on arriving at a certain size, to stop for a while, and send down a root to the ground. As soon as this root was established in the soil, the tree continued increasing to a large magnitude. Here the vital powers of the tree not being adequate, from scanty nourishment, to the usual annual degree of increase in the branches, were accumulated in the root, which therefore was excited to an extraordinary exertion, in its own natural direction, downward. There is no occasion then to suppose, as some have done, that the tree had any information of the store of food at the foundation of the wall, and voluntarily sent down its root to obtain it; nor is it wonderful that the Author of life should provide for it as effectually as it could for itself, had it really been a reflecting being. So in the case of the grasses in question, I presume the herb being in the first instance starved, by a failure of the nutrimental fluids hitherto conveyed by the water of the soil, its growth would be checked, and when checked the same growth could not, as we know by observation on vegetation in general, be instantaneously renewed. A sudden fresh supply of food would therefore cause an accumulation of vital energy in the root, which would consequently assume a degree of vigour and a luxuriant mode of growth not natural to it, and become bulbous. Thus it acquires a resource against such checks in future, and the herb is preserved alive, though in a very far less luxuriant state than when regularly and uniformly supplied with its requisite nourishment. These are not solitary instances. It is well worthy the attention of an intelligent cultivator to seek them out, and turn them to his advantage.
- A corruption, as I presume, of the Italian name Girasole Articiocco, sun-flower Artichoke, as the plant was first brought from Peru to Italy, and thence propagated throughout Europe.
- I have had scaly buds form even on the flowerstalk of Lachenalia tricolor, Curt. Mag. t. 82, whilst lying for many weeks between papers to dry, which, on being put into the ground, have become perfect plants, though of slow growth.
- A particular fact of this kind concerning an ash was communicated to me by the late Rev. Dr. Walker of Edinburgh. See also Trans. of Linn. Soc. v. 2. 268.