This section is from the book "The Native Flowers And Ferns Of The United States ", by Thomas Meehan. Also available from Amazon: The Native Flowers and Ferns of the United States in Their Botanical, Horticultural, And Popular Aspects.
Stem erect, terete, somewhat glabrous or sub-pube-rulous, branched; leaves lanceolate, narrowed at the base, somewhat glabrous, ciliate, entire or occasionally lobed above the middle, the lobes and apex of the leaves tapering to a thick mucro or point; heads few, peduncled; involucral scales linear, subulate, puberulous; ligules in two series, double the width of the disc, white. (De Candolle's Prodromus Systematis naturalis regni vegetabilis, vol. v., p. 285.)
WE have here a plant which has had no popular history, nor hitherto any history at all in connection with the Flora of the United States, for it is only recently that it has been known as growing within our territory; and for this knowledge we are indebted to the botanists of the Cambridge Botanic Garden, Mass., from whence was obtained the specimen from which the drawing was made. As it is not described in any American work at hand, we have taken our description from De Candolle, who first named and described it in 1836 from living plants grown in one of the European Botanic Gardens from seeds received from Mexico. Though with so little already known of it, we are glad to have the opportunity of figuring it, as it offers many excellent lessons both in botany and in beauty. It is of course understood by most students of plants that every part is made up or grows out of some other part; but it is not so well understood, though it is believed to be as true, that one species is made up or grows out of some other species. The knowledge of the first constitutes morphology; the second study is known as the doctrine of evolution. Our present species,
Erigeron mucronatum, the pointed-leaved Erigeron, is well adapted to illustrate both these divisions of botanical science.
As regards the change of leaves to flowers, the manner varies with different species. We like to refer to it in composite or aster-like plants, for it is seldom that we proceed to trace it in any one of them without learning much of the immense amount of variety which nature works out of a few simple materials. It may be remembered that there is not only growth in plants, but that growth is in waves or rhythms, and that there is a succession of these waves - the degree only varying with the species. In the present one, we see that the earliest leaves - those near the root of the plant - are small, slender, and rather blunt. As the growth-wave proceeds, it gathers force, the leaves become larger, and, when at what we may term the maximum force of the growth-wave, deeply lobed. As the force declines, the stems become weaker and are long drawn out, the leaves becoming again smaller, and resembling those at the base or beginning of the movement. The growth-wave is nearly exhausted, and growth almost at a stand-still, when a second and weaker wave begins, and this has more spiral activity than the first, and results in the general calyx of the compound flower, or involucre, as it is technically called, every scale of which is a changed leaf.
Only for the rapid torsion, and for the peculiar position as under the second wave of growth, these now involucral scales might have been leaves drawn out on a stem just as we see in the earliest phase of growth. But a third distinct wave commences when the flower is to be formed, and the ray florets may be compared with the little root-leaves forming the beginning of the first wave as already noted. They are more slender, but there is the same tendency to narrowing at the base, and to obtuseness at the summits. It is particularly interesting to watch these phases of growth in living composite plants. The actual rest between the several waves can in many cases be noted, and often timed.
Each floret in a compound flower is a metamorphosed branchlet with its stem and leaves. The corolla is probably formed of five primordial leaves; generally united into a tube (Fig. 3) in the central or disc flowers, but bursting on one side and looking like a single strap in the ray florets. The peculiar force which accomplishes this rolling up process in the formation of the tube, or in opening the tube to make a strap-shaped floret, whichever it may be, varies in different genera. Usually, the outer row alone is strap-shaped and the rest all tubular, or all may be strap-shaped, or the whole tubular. But a peculiarity of this Erigeron is that there are two series of strap-shaped corollas, while all the rest are tubular as in Fig. 3.
But in some species of plants there are not only transitions of leaves, but transitions in branches, and it is one of the fortunate features of this species that it affords the illustration. Some species of Erigeron have a matted root-stock; others send out thread-like stems, with perhaps a few very small scales, terminating in a bud which eventually becomes a young plant, as in the runner of a strawberry. In the case of the strawberry there are instances known where the runner erects itself and becomes a bunch of flowers with leaves, and finally fruit. We thus know that the thready runner and the flowering branch are essentially the same thing, but, under the action of some peculiar phase of growth force, one has changed its manner and form for the other. In our present species we have, in Fig. 2, a branch just intermediate between its own flower stems, and the thready runners or stolons of other species; and we see how readily the one may be transformed into the other.
Now these peculiar gradations which we find in the individual, and the science of which we know as morphology, exist in the same degree between species, and which then constitute what we call the science of evolution. In these Erigerons it is very well marked. Taking three closely related American species, E. bellidifolium, E. mucronatum and E. Philadelphicum, we have the last with matted root stocks, the first with long thready stolons, and the second intermediate, as it were, having trailing, barren branches, which sometimes root, and at other times are partially erect, as if with a little encouragement they would become flowering branches.
The botanical lesson has been so interesting that little room is left to analyze the plant's beauty. Many of the genus are so coarse in their features that this one surprises by its gracefulness and elegance. It has little to offer in the way of contrast; its claim to distinction lies in its variety of slender lines which all run imperceptibly into each other.
There is much classical history connected with the genus, though not of the species which, in our desire to profit by the lesson it offered, we shall have to pass for the present, hoping to illustrate another species soon.
In English gardens it is erroneously known as Vittadenia triloba.
I. Flowering stems.
2. A trailing, barren stem.
3. Disc flower very much enlarged.