Origin of Cleavage Polyembryony in Conifers

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Abstract
Cleavage polyembryony and simple polyembryony are discussed in relation to developmental selection. Polyembryony in conifers is a highly selective process, one in which the successful embryo must demonstrate its superiority over many other individuals by its own growth activity. This activity may represent a sort of orthogenesis on a mechanical basis, the products of this orthogenetic phenomenon being long, multicellular suspensors, which serve as offensive weapons of the embryonic struggle. In conifers, the selective process is actually a relay race in which the combination of a pollen tube plus its embryo is struggling against other pollen tubes plus their embryos. Discussion shows why cleavage polyembryony is a palingenetic feature of the embryogeny, a condition which had its origin at a time before pollen tubes effected simultaneous fertilization and before the present forms of developmental selection were established. The occasion for the origin of cleavage polyembryony occurred during the period of transition from independent micro-gametophytes to pollen tubes, at the time when independent male gametophytes were beginning to become siphonogamous. For a period cleavage polyembryony was established but later evolution tended to eliminate cleavage of the zygote in many lines so that many of the conifers have more recently returned to simple polyembryony. That cleavage polyembryony was an episode in the history of conifer evolution, one through which most, if not all conifers have passed, is shown by certain anatomical evidence[long dash]rosette embryos, rosette cells, etc. Such structural evidences are presented and illustrated. Conclusion is therefore that cleavage polyembryony, though retained in some conifers as a specialization, is essentially a primitive feature of the embryogeny.

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