396 FOURTH GROUP. SEED-PLANTS. 



but passes in the latter during germination. The occurrence of ripe seeds with or 

 without endosperm is more or less constant within large groups of plants and is 

 therefore of systematic value; among the better-known families, for example, the 

 Compositae, Cucurbitaceae, Papilionaceae and Cupuliferae (Oak, Beech) have seeds 

 without endosperm. Sometimes the embryo grows within the seed to such a size, 

 that the endosperm is only like a rather thin membrane surrounding it. 



To return once more to the recently formed oospore ; in Angiosperms as in 

 Gymnosperms it is not as a rule at once transformed into the embryo ; the end which 

 is towards the micropyle becomes attached to the wall of the apical convexity of the 

 embryo-sac, and the free extremity is directed towards the base of the ovule ; it then 

 elongates and undergoes in doing so one or more transverse divisions. The embryo 

 is usually formed from the two terminal cells of this row of cells and the suspensor 

 from the others \ 



The embryos of Alisma Plantago and Capsella Bursa-pastoris which have 

 become well known since Hanstein's researches may serve to illustrate the develop- 

 ment of the embryo. We desire to know how and where the first organs of the 

 embryo (the radicle, \heplu?nule and the cotyledons} are formed, and how the dermatogen, 

 the periblem and plerome are differentiated. The first point to notice is, that the root 

 (extremity of the radicle) is always formed at the posterior extremity of the embryo 

 which is towards the point of attachment, and that the plumule is lateral (Monocoty- 

 ledons) or terminal at the free extremity which is remote from the point of attachment. 

 Capsella affords the best illustration of the development of the embryo in Dicotyledons. 

 The oospore first elongates considerably and assumes the form of a tube, the upper 

 or micropylar end of which is divided by a number of transverse walls. The 

 embryo is chiefly formed from the terminal cell of this row of cells. Three stages 

 may be distinguished in the development of this cell ; in the first it becomes 

 spherical in form without external differentiation, while the different layers of 

 meristem are already separated from one another in its interior; in the second 

 stage the embryo becomes differentiated into radicle, stem and cotyledons, and in the 

 third it grows in all its parts into the state in which it is ready for germination. The 

 development of the embryo begins with the increase in size of the terminal cell, which 

 then divides into halves by a radial wall (Fig. 326 /, i). As commonly happens 

 in the case of cells developing into organs which are nearly spherical in shape, for 

 example in the embryos of the Vascular Cryptogams, a second longitudinal wall 

 follows the first and at right angles to it (lying in Fig. 361, 7 in the plane of the 

 paper), and then a transverse wall at right angles to the two former walls (Fig. 

 326 /, 2), so that the small embryonic sphere is now composed of the octants of 

 the sphere. 



The cotyledons and the plumule or epicotyledonary portion of the young 

 plant subsequently proceed from the upper half of the embryo which is cut off by the 



1 The embryonic body in the condition before the differentiation into suspensor and embryo 

 is termed the pro-embryo. On the development of the embryo see Hanstein, Entwicklungsgesch. d. 

 Reims der Monocot. u. Dicot. in his Bot. Abh., I. Bd. Westermaier, Capsella bursa-fastoris (Flora, 

 1876), and Hegelmaier in Bot. Ztg. 1874, p. 631. Koch, Orobanche in Pringsheim's Jahrb., XI 

 Bd. p. 218. Famintzin, Embryol. Studien (Mem. de 1'Acad. imp. de St. Petersbourg, Ser. XXVI). 

 See also below. 



