EMBRYOLOGY OF PLANTS. That phatse in the life history designated as the em bryology begins within the fertilized egg, but its end is not marked by any such definite feature. In general, the embryo represents the early stages in the development of an individual from the egg. In the ferns and their allies, some what later stages, in which one or more leaves are visible to the naked eye, are called spore lings. There is no definite feature to mark a line between the sporeling and the adult plant. In the seed plants, the series is embryo, seed ling, adult, with no features to mark the tran sitions. The difficulty is the same as that in de fining baby, boy and man. The early stages in the development of the embryo are fairly well known in all groups from the liverworts to the highest floweringplants. In the liverworts and mosses, the development of the embryo from the fertilized egg up to the adult stage, and even to the death of the individual, is rather short. The embryo, and even the adult, are small, are parasitic upon the egg-bearing plant (gametophyte), and do not produce any leaves. In the lowest liverworts the egg divides into halves, then into quarters and continues divid ing until a spherical mass becomes differentiated into an outer protective layer enclosing a large number of spores. In the higher liverworts and in the mosses, the embryo starts in the same way, but later becomes differentiated into three regions called the foot, stalk and the latter producing the spores. In the lower liver worts, the adult is a small spherical body not more than one-sixteenth of an inch in diameter; in the higher liverworts and in the mosses the diameter is not much greater, but there is con siderable elongation. A couple of inches is rather long; but a few liverworts reach a length of five or six inches and one of the higher mosses is said to reach a length of 10 or 12 inches. (Figures of some of these features may be found under SPoauearrx, Evourriox or). In the ferns and their allies, the embryo begins to develop in the same way, forming a spherical mass of cells, but definite growing re gions soon appear, marking the root, stem, leaf and foot. The embryo is parasitic upon the gametophyte until the root becomes developed and begins to get nutrition from the soil and the leaf begins to secure materials from the air. When this stage has been reached, we no longer call the young plant an embryo, but a sporeling. In the seed plants, which include the Gymnosperms and Angiosperms, the develop ment of the embryo presents great variation and complexity. In the cycads (q.v.) which rep resent the lower living Gymnosperms, the fer tilized egg does not immediately give rise to a mass of cells, but nuclear divisions, without any separating walls, take place, until there may be as many as 1,000 nuclei lying free in the cyto plasm of the egg (Fig. 1, A). Cell walls then appear at the lower part of the egg (Fig. 1, B). The cells, thus formed, become differentiated into three regions, (1) a group of cells remain ing within the limits of the egg, (2) a region of rapidly elongating cells called the suspensor and (3) at the tip of the suspensor some small cells with dense protoplasmic contents (Fig. 1, C). The root, stem, cotyledons and leaves of the embryo come from these small cells, the other two regions being temporary structures which function only during the early develop ment. After the embryo breaks out from the seed and becomes independent, it is usually called a seedling. The eggs of the cycads are very large, reaching one-eighth of an inch or even one-fourth of an inch in length. In the higher Gymnosperms, the eggs are much smaller, in most Pines not more than one-one hundredth of an inch in length. In these higher forms there is a constant tendency to reduce not only the size of the egg, but also the num ber of free nuclei. There are still the three re
gions mentioned above, but each consists of only a few cells. In a few Gymnosperms, the free nuclear period is entirely eliminated, a cell wall following the first division of the egg nu cleus. In the Angiosperms the eggs are still smaller, all being microscopic in size, and in all the cases the first division of the nucleus of the fertilized egg is followed by the formation of a cell wall so that there is no free nuclear stage. Even under the microscope, the eggs of this group look so exactly alike that it hardly seems possible for one to develop into an herb, another into a shrub and another into a tree. We say the course of development is determined by heredity, and those who are satikfied with the mere naming of a phenomenon may be satis fied with this explanation. Although the eggs and embryos are very small, modern technic is so efficient that the embryology is well known from the willows and crowfoots to the sun flowers and orchids. A simple and fairly typi cal type of embryology is illustrated by the Shepherd's Purse (Capsella), a familiar and widely-distributed weed (Fig. 2). The first di vision of the fertilized egg is transverse (A). Divisions then take place so that a filament con sisting of a single row of cells is produced (B) ; the terminal cell of the row then divides vertically and from the two resulting cells the stem, cotyledons, leaves and nearly all the root are produced (C). The cell in which the verti cal division has appeared is generally called the embryo cell, and the rows of cells below it, the suspensor. A second vertical wall at right an gles to the first one gives rise to four cells, each of which immediately divides transversely, so that eight cells, apparently just alike, arc produced (D). Each of the eight cells now divides, forming a wall parallel to its outer sur face (E). These outer cells (dotted in the il lustration) continue to divide, but all walls are perpendicular to the surface, so that the result is an extensive layer of cells only one cell in thickness. Since this layer, at maturity, is the epidermis, it is called the dermatogen, which means the epidermis producer. In the lower half of the more or less spherical embryo, the four central cells, inside the dermatogen, divide longitudinally (F). The four inner cells re sulting from this division (dotted in the illus tration) constitute the plerome and give rise to the vascular system of the root; the outer four give rise to the periblem which gives rise to the cortex of the root. In the upper half of the embryo, which is to form the stem and leaves, the differentiation into cortex and vascular re gion takes places much later, after a large number of cells has been produced. Thus there are three embryonic regions, one producing epidermis, another producing the vascular sys tem and the third producing cortex. These three regions, established in the early develop ment of the embryo, are also found in the adult plant.
There are other types of embryology in the flowering plants. Many have no filamentous stage; some have a single, very large suspensor cell, while some have a massive suspensor. In many the differentiation into the three embry onic regions takes place much later; some do not differentiate at all until the seed germinates; while in others, like the bean, the embryo, while still in the seed, has not only cotyledons but well-developed leaves. Some special features of embryology tvill be found under PLANTS, RE CAPITULATION IN, and SPOROPHYTE, EVOLUTION OF. Consult (Morphology of Gymnosperms,' by John M. Coulter and Charles J. Chamber lain; 'Morphology of Angiosperms,' by the same authors; (College Botany,' by G. F. At kinson; (Mosses and Ferns,' by D. H. Camp bell. CHARLES J. CHAMBERLAIN, Professor of Cytology and Morphology, Uni versity of Chicago.