In Darwinia the floral parts are differentiated in a "calyx-orolla-gynoeciumandroecium"
sequence. In individual buds stages of microsporogenesis markedly precede
corresponding stages of megasporogenesis. The anther is tetrasporangiate with all
sporangia lying in one plane. The secretory tapetum is one- to three-layered within the
same microsporangium and a large number of Ubisch bodies are formed. The anthers
dehisce by minute lateral pores and an ingenious mechanism helps disperse the twocelled
pollen grains.
A basal placenta in the single loculus of the ovary bears four ovules in D. micropetala
and two in D. fascicularis. In both species, however, only one ovule is functional
after fertilization. The fully grown ovules are anatropous, crassinucellar, and bitegmic;
the inner integument forms the micropyle. The parietal tissue is most massive at the
completion of megasporogenesis but is progressively destroyed later. The embryo sac
follows the Polygonum type of developnlent and when mature is five-nucleate, the three
antipodals being ephemeral.
Following fertilization, the primary endosperm nucleus divides before the zygote.
Subsequent nuclear divisions in the endosperm mother cell are synchronous and lead to a
free-nuclear endosperm which becomes secondarily cellular, starting from the micropylar
end at the time the globular embryo assumes an elongated shape.
Embryogeny is irregular and the mature embryo is straight with a massive radicle
and a hypocotyl which terminates in two barely recognizable cotyledons. Sometimes the
minute cotyledons are borne on a narrow neck-like extension of the hypocotyl. A
suspensor is absent.
Both integuments are represented in the seed coat and only the outer layer of the
outer and the inner layer of the inner integuments, with their thick-walled tanniniferous
cells, remain in the fully grown seed. The ovary wall is demarcated into an outer zone
containing oil glands surrounded by cells containing a tannin-like substance and an
inner zone of spongy parenchyma. In the fruit this spongy zone breaks down completely
but the outer zone is retained.
The two species of Darwinia, while closely resembling each other in their
embryology, differ significantly from other Myrtaceae. However, no taxonomic
conclusions are drawn at this stage, pending enquiry into the life history of other
members of the tribe Chamaelaucieae.
Reproductive development and fertility traits among heifers in different residual feed intake groups1
