The Botanical Gazette Volume 73 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1922 edition. Excerpt: ...leptosporangiate species growing near by. Fig. 24.--Embryonic selection in Aspidium: A, enlarged view of larger embryo of C, surrounded by calyptra; B, smaller unicellular embryo shown in C on same gametophyte, less than i mm. distant; (C reconstructed from several serial sections); A and BX2oo, CX42. This is included here because it seems to show a stage slightly later than that of fig. 24, and indicates the fate of the smaller embryo of the latter. It is especially interesting to note the shrunken and starved appearance of the embryo in fig. 25 B. That the larger embryo starves the smaller is a very natural explanation; this is a factor which is very certain to be involved, but it is also possible that the excretions of one embryo tend to inhibit the development of the others. In ferns having large vigorous gametophytes with many archegonia, if the aborted embryos are not too quickly starved they should be subject to recall experimentally, by any measures which would tend to prevent this embryonic competition. From a careful examination of a number of species of leptosporangiate ferns, it is clear that there is a considerable period of enlargement of the egg following fertilization, before the zygote divides. It is obvious that the decisive part of the embryonic selection may transpire during this early period, so that most if not all of the zygotes are eliminated before they have undergone cleavage. A few examples may be given showing the amount of increase in volume during the first stages of the embryogeny, calculated Fie. 25.--Fern gamctophyte with several embryos in competition, representing slightly later stage than fig. 24; A is d enlarged; B is e, (second unicellular embryo) with disintegrating nucleus, collapsing as it is aborted...