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tuberous bitter pea

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tuberous bitter pea - TUBEROUS BITTER PEA (Lathyra macrorrhizus)
tuberous bitter pea - In the Tuberous Pea we find emphasised one of the most striking characteristics of a mighty clan. In meadows and hedgerows, on commons and under thickets, the great tribe of the Peas and Vetches are found. They are a hardy race with strongly marked family resemblances and but minor points of difference; indeed, the only general distinctions we can draw between them are - as to leaves - those of the Pea are fewer and larger than those of the Vetch; as to flowers - the Pea petals are somewhat broader and the Pea ovary-column somewhat more flattened than the Vetch petals and Vetch column respectively. And since these qualities are but relative and little enough to guide one unfamiliar with every species, it is often difficult to say off hand whether a newly-found specimen is a Pea or a Vetch.

The subject of the illustration is the Tuberous Bitter Pea, which is common enough throughout the country in somewhat protected situations, such as under a thicket or a hedge, or creeping about in some open woodland. It gets both the "tuberous" in its English name and the "macrorrhizus" in its botanical name from the fact that its roots are knotted in a peculiar and excessive way, for all along the creeping rootstock that persists year after year little tubers are to be found, some as large as a pea, some as large as a cherry. Now, root nodules of some description are common to all the plants which carry the butterfly type of flower - the family of the Leguminosæ - but nodules so large as these are only found on one or two others, hence the opening statement of this chapter. They are full of starch, and when cooked are said to be both nutritious and palatable, with a flavour like liquorice. It is stated that after being dried in the sun they were the base of a substance called "cormeille" which was chewed by the Highlanders as a preliminary to enjoying their whisky with greater gusto. It is further asserted that, after being roasted like chestnuts, they have often served as food, and it is suggested that they were the substance upon which Cæsar's famished soldiers once fed when otherwise they would have perished of starvation. But there is somewhat of a confusion as to whether the Pea-tubers that indubitably can serve as food are those of this plant, or of a very near relative, a doubtful native of this country, viz. the tuberous vetchling or earth-nut pea. Probably both plants have, at times, furnished these earth-nuts.

But the real point of interest to us to-day is the wonderful discovery that they are more than they appear on the surface, more than mere starchy tubers, like a potato. Now, it has been realised from very early times that peas, vetches, beans and such-like plants enrich the ground in which they grow, while other plants impoverish it, though why this was so was a mystery. It was left to a German professor, Professor Hellriegel, to point out, in 1886, the solution of the riddle. He showed that these nodules and tubers were the dwelling places of bacteria, which bacteria are able to tap that great storehouse of nitrogen, the air, work up the nitrogen into a form in which the plant can assimilate it, and then pass it on to the roots. No plant without the aid of these bacteria can deal with the free nitrogen of the air. Hence, while the amount of nitrogen which is present in ordinary crops - such as wheat - which are without root nodules, can be accounted for by the nitrogen they have obtained from the soil, water, and manure, the amount of nitrogen in a crop, say, of peas or clover, exceeds considerably all that they could possibly have obtained from these sources. But the benefit is by no means one-sided, for the bacteria themselves live sheltered in the nodules' tissues, so that the whole arrangement is one of mutual benefit. The increased size of the root where these colonies dwell is due not to the size of the bacteria, which are infmitesimally small, but to the inflammation of the tissues this invasion brings about. We may, therefore, regard the tuberous roots of this pea as special reminders of one of the romances of science.

The stem of the plant is weakly, often lying partly on the ground. Down between the leaflets it is slightly winged. These wings assist the leaves in their work for the plant; indeed, in another pea plant - the grass pea - they have entirely superseded the leaves.

The leaves consist each of two or three pairs of leaflets, sometimes almost oval, sometimes long and narrow, but, instead of the central leaf-stalk ending with tendrils or the usual single leaflet, it terminates merely in a point just beyond the top leaflets. This particular species of Pea only grows from six inches to a foot high; therefore it has no need of tendrils. Sometimes the point develops into a very narrow leaf, showing that originally it followed the fashion of its family. Just where the leaf is attached to the main stem there are a couple of paler green leaf-like structures - the stipules - shaped like arrowheads, the chief function of which is to wrap up the leaf when it is in the bud stage. Also they provide barriers to prevent ants and other small insects crawling up the stem to the flowers.

The flowers shade from purple to red, the younger ones being the pinker; they fade into blueness. They are fairly conspicuous and are in loose clusters, three or four together, and are of the butterfly-type, with a large upstanding petal - the standard - two smaller side ones - the wings - and two still smaller which are joined together to form the keel; if these be looked at in profile they will be seen to be remarkably like the keel of a boat. This corolla is set in a small green sepal cup with five points, its lower part being longer than its upper. Lying inside the keel and shaped to it are the stamens, ten in number. A tube is formed by the union of their lower halves, but since the top stamen lies loose and does not join in with the rest, all along the top of the tube is a narrow opening. Honey is hidden away on the inside of this tube. In the centre of the flower is the pod in miniature clothed with silky hairs; its top projects into a curved column, and at the tip of this is a brush of fine hairs. The anthers open and are ready to pour out their pollen as soon as the flower uncloses.

Attracted by honey and scent a bee comes and settles on the wings or keel. His weight presses down the keel, and the stamens are uncovered. By the same movement, the column of the pod, tipped with the brush, sweeps through them and swiftly carries out the pollen-dust to the insect's under side. Since all the pollen is not swept out each time, the same brushing-out may happen three or four times as successive visitors arrive, and the flower thus uses several emissaries to carry its pollen away. The bee, of course, comes after the honey in the tube, and the slit-like opening allows it easily to insert its tongue and obtain it. The next flower it visits gets fertilised by its dusty abdomen being pressed against the stigma as this brushes forward.

The pod-like fruit is particularly conspicuous in this species, as it is long and drooping, and eventually turns quite black.

It is a curious characteristic of this plant that when dried and pressed it turns completely black, as if there were a suggestion of parasitism about it.

There are ten different kinds of Pea and ten different kinds of Vetch among our native wild flowers.

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