Popular Science Monthly/Volume 25/September 1884/How the Dodder Became a Parasite
OVER yonder in the corner of a field there grows a mass of yellow threads, looking at a distance like an immense spider's web covering a number of plants. Closer inspection reveals it to be the dodder, poetically called by some the golden-thread. Though beautiful in the abstract, handsome in its golden color, it is yet a vile and pernicious weed—one that in the flax-fields of Europe in one form, and in the alfalfa-fields of California in another, has done a vast deal of harm. Yet it is, to look at, beautiful. The flexuous stem of golden yellow, adorned with clusters of white, bell-shaped flowers, twining among and over other plants, forms a striking contrast with their green stems and leaves. And it is no wonder it has been sometimes cultivated for its beauty. Why, then, should we call it a pernicious weed? Look closer, and you will see that at intervals along the stem, where it clings closely to other plants, it has sent out bunches of little rootlets, which, not content with performing the office of hold-fasts, force their way through the bark, penetrate the tissue, and take the matter found there into their own systems.
Still closer examination will reveal other features. In the first place, there are none of the green leaves usually found on plants. Secondly, there is no root fastening the plant to the soil. Why is this? What is the reason that this plant grows and flourishes like other plants, and has yet neither root nor leaves? Let us see.
What is known as parasitism in plants is not confined to any one family or class. Various orders have one or more genera with species which take their nourishment in a complete or partly elaborated form from other plants. Sometimes they are perfect parasites, and take everything they need from other vegetable forms. Sometimes, as in the mistletoe, they take the partially made sap, and complete its transformation within their own tissue; while in still other instances only a very little of the sap is taken, and the other nourishment is absorbed from the soil by the roots proper.
Our dodder is an example of a perfect parasite. All the material necessary for its growth it takes ready made from the plants upon which it grows. As the purpose of leaves in all plants is to prepare from the crude materials in the air and soil the matter necessary for its growth, and as the dodder finds and appropriates this material already made, the absence of leaves is at once accounted for. There was no need for them, and they ceased to be.
The want of a root is another matter. When the seed of the dodder is examined, it is found that there is simply a coiled embryo, with very little albumen. The usual seed-leaves are absent; so that, for its first growth, it must depend entirely upon the albumen in the seed. When this seed first germinates, a little rootlet penetrates the ground. Owing to the deficiency of food, it only exists long enough to enable its stem to grow till it reaches some plant upon which it can fasten. When this is accomplished, the young plant will grow rapidly, and soon sever its connection with the ground; but, if not able to reach some support, it dies entirely.
In order to comprehend the reasons for the peculiarities of the dodder, and understand how it came to assume its habit of complete parasitism, it will be necessary to notice the probable rise and progress of the habit. We can do this by looking at some of those plants which are not yet such complete pensioners on the bounty of others. For it very seldom happens that all the steps leading from a normal to an out-of-the-way mode of living are lost. Some few will remain, to indicate the line along which the plant has proceeded. Imperfect adaptations point surely the path leading to perfect development.
The modes of living of the dodder and the Indian pipe may be considered as the two extremes of one line of development. The first is a complete parasite, and the second has gone so far as to become a saprophyte. The central point from which sprang the two branches is probably represented in certain species of Gerardia. Here is found the first indication of the parasitic habit. While the roots are attached to those of other plants, its green leaves are well developed, and it takes only the crude material into its system and there elaborates it; and at the same time it absorbs matter by means of the other roots with which it is provided.
The mistletoe comes second. In this plant we find the root absorbing nourishment from the branch on which it has located; the stem provided with green leaves, to which it can bring the sap to a proper state for assimilation, but no connection with the soil. The next step would be for the plant to loose its connection with earth or branch, take the fully elaborated sap, and by gradual stages lose all its foliage organs. Then the fully formed parasitic dodder results.
Proceeding in the opposite direction we find the beech-drop, a plant which lives in the rich mold of beech-woods, taking part of its food from the decaying leaves, and part from the roots of the beech-trees which it penetrates with its own rootlets. This plant is entirely destitute of green leaves, is of a brownish color, and may be considered one step on the road taken by the Indian pipe.
The Indian pipe, again, is a little plant which lives in the débris of forests, finding its food in the mass of decaying vegetable mold. While it is not probable that its roots are connected with those of the trees under which it grows, it is certain that the rich matter there found contains the constituents it requires for its growth. It, like the dodder, is destitute of green leaves, and for the same reason, namely, because it finds its food already prepared for it and has only to absorb it. But it differs in taking the food from the dead and decayed matter, instead of from the living. Plants of this kind are known as saprophytes, and are most common among the fungi. Here, then, in the saprophytic Indian pipe we have one end of a line of habit of living which has its other end in the perfect parasitism of the dodder.
In attempting to trace the origin of any particular habit peculiar to any one species, it is always necessary to examine the near relatives and see in what respects they resemble and in what ones they differ from the plant under consideration. The dodder belongs to the Convolvulaceæ, or the morning-glory family, and one of the most striking features of this family is found in their habits of twining. But what a vast difference there is in appearance between the morning-glory, with its large leaves, its root, and its conspicuous flowers, and the dodder, with its yellow stem, complete absence of green leaves, and lack of root! How is the change to arise which will bring the dodder to its present condition?
Evolutionists acknowledge that all changes in either plants or animals are the results of changes in conditions or surroundings. When once a change has occurred which is beneficial in a certain way, the probability is, that the plant or animal will continue to develop in that direction till it diverges widely from the original form. The struggle for existence will cause all the imperfect forms to be killed off, and only those will survive which are best suited to the altered conditions of life. Once let an organism begin to vary in any one direction, and there is no telling where or when it will stop. This much is certain, that it never ceases until the best results possible have been attained.
The chief characteristic, then, of the convolvulus family is the climbing habit. The origin of this habit is found in the fact that sunlight and air are two things needful for a plant's proper growth and development. In situations where these two things are found in limited quantities, plants with climbing habits and animals with arboreal instincts will abound. In Brazil, for instance, where immense tracts are covered with a dense forest-growth, it is noticed that all forms of animal life have become adapted to residence in trees. Many of them live there entirely. Monkeys seldom leave the tree-tops. Lizards and snakes and insects are there, and even man himself is often found living among the branches. So, too, plants form immensely long stems, reaching in many cases to the tops of trees a hundred feet high. The extraordinary development of climbing powers has been gradually acquired in the course of ages. In times and places where vegetation was not dense, and where the struggle for light was not great, plants of erect habit succeeded well. Then it was a conflict to see which could grow tallest. But when a weak plant found that, by taking hold of its tall and erect neighbor and by clinging to it, it could reach the sunlight much easier and by an expenditure of much less material than by growing erect itself, it was a great step on the road. This habit, being transmitted from one generation to the next, kept on improving. Less and less rigid, more and more flexuous stems ensued, and the delicate climbing vines of modern times are the results of this necessity of reaching sunlight with as little waste of material as possible.
There are many methods adopted by plants to climb. While some of them reach upward by means of tendrils developed at the ends of stems or leaves, others twist their petioles round the support, and still others twine their stems round other stems that may come in their way. This last is the method adopted by those of the Convolvulaceæ which climb at all. For even in this family there are some species which are erect in growth. The Calystegia spithamœa is one of them. Others do not grow up into the air, but trail along the ground or over low plants, and thus secure their due share of sunlight. Others, again, climb freely, and this is the case with the dodder.
The climbing bitter-sweet is said to sometimes strangle the trees upon which it grows. The constriction caused by its growing stem is so great as to cut off the supply of sap from the roots, and cause the death of the tree which has supported it. The original ancestor of the dodder was a plant with a well-developed root, green stem and leaves, and a twining habit. If its clasping killed the stem which supported it, the effect would be disastrous, for then it would not accomplish the purpose of its climbing. If the twining stem sank into the supporting one, it might cause decay along the line. This decaying would tend to develop rootlets from the side of the climber. The rootlets, used at first merely to assist in climbing, might and must have become modified so as to penetrate the bark to the tissue beneath. A minute absorption of the sap from this would be an assistance. Gradual increase of the amount absorbed would lead to gradual increase in the number of rootlets. And, this continuing, less and less need would be felt for the leaves. As needless organs are sure to degenerate, the leaves would become smaller and smaller, lose more and more of their green color, and finally become the yellow scales and bracts they now are.
Along with the loss of the leaves would go the root. Becoming less necessary, it would get smaller, until finally it would retain only enough of its original character to give the plantlet a start in life, and transmit its qualities to its progeny. Of course, all these changes would be made slowly; but they would come surely. If each succeeding generation of rooting stemmed plants throve better in any way, perfected seed in any greater abundance, or were enabled to crowd out competitors in the struggle for life, we may be sure that the descendants of the favored plants would inherit these good traits, and would send more and more rootlets into the enveloped stem, until at last the habit would become firmly fixed. Thus would be formed a leafless, rootless parasite, so well adapted to hold its own that it would probably exterminate some of the less favored forms.
The commencement of the habit of sending rootlets into stems has been observed in occasional specimens of the convolvulus. Let but this habit grow and be improved upon, as it surely will be if it is found beneficial, and from this small beginning we can look for just such a development as has been found in the dodder. It can not be said that there is always an upward progress in Nature. Degenerate forms exist and thrive as well as regenerate ones. The truth is, that when a plant or an animal can fill a vacant space in the world better by going backward than by going forward, the retreat is sounded. Progress or retrogression, it is the same. The direction best suited to Nature's needs is the one taken; so that, while on the one hand there may be a wonderfully complex organism, perfectly fitted for the struggle for life, on the other hand there may be a very degenerate one equally fitted into its place.