Structure of Epigaea repens.
The "first sweet smiles of May," as Whittier calls the flowers of this plant, prettily portray the fact that they are at once among the earliest and the most prized of our spring floral treasures.
In most localities where the plant is known it is called the Trailing Arbutus, but in Massachusetts and some other portions of New England it is commonly called the Mayflower, partly, perhaps, in allusion to its time of blossoming, and partly to the fact that it was the first flower to gladden the eyes of the Pilgrims in the spring after their first winter on the bleak shores of their new home.
The plant is widely distributed over the northeastern part of North America, but is especially abundant in the region of the Alleghenies and in the pine and fir-clad regions bordering the Great Lakes and the St. Lawrence River. It particularly affects rocky hill slopes, where the soil is light and well drained, but is not infrequently found on lower-lying sandy, tree-clad areas, where the soil is well aerated. The sturdy little plant also prefers the vigorous north, and is seldom found south of the line of the Ohio River, save in the more elevated portions of the Alleghenies. Its stems are prostrate, and the slender trailing branches often attain a foot in length, and possess, like the petioles and the under surfaces of the leaves, a copious growth of rusty-brown, multicellular, but simple hairs. These also occur, but much more sparingly, on the upper surface of the leaves. The leaves are evergreen, veiny, scarcely coriaceous, deep green above, from 3 to 6 cm. long, elliptic in outline, ciliate-margined, cordate, or sometimes rounded at the base, cuspidate at the apex, exstipulate, but provided with petioles which are nearly as long as the leaf blades.
The fragrant flowers are arranged in short, almost spike-like racemes at the ends of the stems, and when they unfold in the spring are often quite concealed from view by dead leaves which have fallen from the overhanging trees. The flowers are white or rose-tinged, and attain a length of 1 ½ or 2 cm. The pedicels are only two or three mm. long, and covered with rusty-brown hairs, as are also the scaly bracts which subtend the flowers. The calyx is deeply five-parted and the segments are erect, lanceolate, entire, nearly smooth, about the length of the corolla tube, pointed and scale-like. The corolla is hypogynous, salver-shaped, and the lobes of its five-parted limb are ovate, entire, obtuse or mucronate, and alternate with the segments of the calyx. The tube is hairy on its interior. The androecium consists of ten stamens, as in most other Ericaceae, and they appear to be in but one whorl, though probably this is the result of a condensation from two.
The flowers, according to the investigations of Prof. W. P. Wilson, are really dioecious, though most of them still possess both stamens and pistils. In the pistillate flowers, which are rose-colored, the stamens have sometimes completely disappeared, though in most instances they are still present, but functionless and smaller than they are in the staminate white flowers. The staminate flowers differ from each other also, some having long stamens, others short ones, and still others those of intermediate length. There is a corresponding difference also in the length of the styles. The history of the flower, then, according to Prof. Wilson, is as follows: It was at first hermaphrodite, and the flowers of the species all had the same form. It then became dimorphous, later on trimorphous, and finally the stamens in some flowers and the pistils in others became abortive, as is the case with the species at the present time. It may therefore be safely predicted that in the process of evolution still going on the last vestige of stamens from the pistillate flowers, and of pistils from the staminate ones, will ultimately disappear.
The insertion of the stamens is, as in most other Ericaceae, on the receptacle, and not on the tube of the corolla, differing thus from most other gamopetalae, in which they are adnate to the tube of the corolla. The filaments are bearded at the base and alternate toward the apex. The anthers are introrse, versatile, 2-celled, and differ from those of the majority of the family in the fact that they dehisce longitudinally rather than by means of apical pores.
The pollen grains also differ from those of most other plants outside this natural order in the fact that each is composed of a group of four cells.
The pistil is 5-carpeled, the ovary faintly 10-lobed exteriorly, 5-celled interiorly, with an axile placentation and very numerous ovules. The style is erect, unbranching and crowned, in the pistillate flower, with a star-shaped, 5-rayed stigma. The stigmas of the staminate flowers are also 5-lobed, but the lobes never open.
A study of the cross-section of the stem shows such a structure as that drawn in Fig. 2; a small-celled epidermis, a loosely-arranged cortical parenchyma, whose cells vary greatly in size, a zone of well-developed bast-fibres in the outer phloem, a narrow zone of wood with a rather large pith composed of parenchyma cells, some ot which are small or moderate in size, others relatively very large.
A cross-section of a leaf near its base shows the vascular area constituting the midrib to possess an outer crescent-shaped mass of bast-fibres, the horns of which are presented toward the upper surface of the leaf. Lining this is a thinner crescent of soft bast, which in turn includes a short, thick crescentic mass of xylem tissues. The latter shows a distinct radial arrangement of its elements, and these rays focu.s upon a small area of parenchymatous tissues included between the horns of the inner crescent. This parenchyma, the xylem tissues, and the bast-fibres, in the mature leaf, are all strongly lignified. Exterior to the crescent-shaped area of bast-fibres is a region of loosely-arranged parenchyma enclosing the bundle, except on its upper side, and extending nearly to the epidermis below, and laterally to form the spongy parenchyma of the thin portions of the lamina. This parenchyma is quite similar in appearance to that composing the pith of the stem.
The epidermis is one-layered and rather small-celled, and the cells of the lower are smaller than those of the upper epidermis. The cuticle is thinner than that of most evergreen leaves. The rather compactly-arranged chlorophyll-bearing cells which face the upper epidermis differ from most palisade tissue in being but slightly lengthened. There are two or three layers of these cells.
The ordinary epidermal cells, in surface view, appear .=trongly wavy in outline, or lobed, and there is little difference of shape between those of the upper and those of the lower face of the leaf. Stomata occur also on both surfaces, but are more abundant on the lower.