Polarity 127 



In seed plants the embryo has a definite orientation in the ovule, the 

 tip of the young radicle always being directed toward the micropyle 

 and the plumular end toward the chalaza. This has its origin in the polar 

 relation between embryo sac and ovule, since the archegonium, or egg 

 apparatus, lies at the micropylar end of the sac. Even the group of four 

 megaspores is polarized, and it is the one at the micropylar end that 

 germinates into the female gametophyte. The planes of division of the 

 proembryo are related to the axis of the ovule. In the young embryo as 

 it develops at the end of the suspensors, the distinction between root 

 and shoot begins very early, with the first transverse divisions. The direc- 

 tion of the polar axis is evidently impressed upon the embryo, as upon 

 the egg, by the axial organization of the embryo sac and ovule, and 

 once established this polar behavior persists and is apparently irreversible. 



Fig. 6-6. Young embryos in the female gametophyte of Isoetes which have developed 

 in the positions indicated. Early orientation is with reference to the polar axis of the 

 archegonium, but when the leaf begins to push out it becomes negatively geotropic. 

 Z, direction of zenith; R, root; F, foot; L, leaf. (From La Motte.) 



Embryonic polarity, however, may arise in other ways than through 

 this simple relationship to the ovulary axis. In cleavage polyembryony 

 several embryos may arise from a single egg (p. 206), each showing typical 

 polar character. Adventitious embryos are sometimes formed by growth 

 of nucellar cells and not from fertilized eggs, and these grow into normal 

 plants. Structures essentially like embryos sometimes occur elsewhere in 

 the plant ("foliar embryos" of Kalanchoe, p. 254) and these show typical 

 polar behavior. The first manifestation of differentiation in any embryo, 

 whatever its origin, is the appearance of a polar axis. 



A number of cases have been reported (Swamy, 1946) in which the 

 polar character of the angiosperm embryo sac is reversed, an egg ap- 

 paratus appearing at both ends, or even the antipodal cells at the 

 micropylar end and the egg at the opposite one. 



Tissue Reorganization. Various histological changes occur in cuttings 

 grown in an inverted position, as described by Vochting ( 1918 ) and 

 others. Such plants are evidently abnormal in a number of respects. 

 There is often a tendency in them to form swellings and tumors, par- 

 ticularly near the insertion of branches, which now tend to grow upward. 



