EMBRYOLOGY IN PLANTS. The study that deals with the earliest stages of plants. In general the embryo arises from the germination of a spore. In ordinary usage, however, the term embryo is applied to the young plant which is deNaloped within the seed. This embryo is the result of the germination of a fertilized egg. and it is definitely limited by the fact that it passes into a period of rest protected by the seed coats.
IN Mossr.s. In mosses the fertilized egg rests in the female organ (archegonium), which is at the apex of a leafy stem. This fertilized egg begins to germinate at cane, and its first wall, re sulting in a two-celled embryo, is at right angles to the long axis of the arehegoninin. The two cells thus formed proceed upon different lines of development ; the lower one produces a de seending structure called the 'foot,' which pene trates into the tissue of the leafy stem and anchors the embryo; the upper cell produces the shoot, which is differentiated into stalk and cap sule. In this case, therefore, there is no strict point at which the structure may be regarded as ceasing to be an embryo, the whole development being emit innons.
Ix FERNS. Among the ferns the arehegonium in which the fertilized egg occurs is situated upon the under side of the small prostrate sexual plant (prothallium). In ordinary eases the fern
embryo is formed in a somewhat, different man ner from that described for the mosses. The first wall of the egg, resulting in a two-eelled embryo, is parallel with the long axis of the arellegoninni. the two cells thus formed facing tire apex and base of the prothallimn. Each of these cells immediately divides, resulting in the o called 'quadrant stage' of the embryo. Each of these quadrant veils is destined to develop a definite body region. The two cells which face the apex of the prothallium develop the stem and leaf structures; those which face the base of the prothallium develop the root and foot structures. The first wall of the fertilized egg, therefore, has had the same general result as the first wall of the egg in the case of mosses; that is, it has resulted in separating what may be called the root and shoot structures. In the ferns the development of the young plantlet is continuous, so that the embryo is limited by no special point in time or structure.