Post-Fertilization Events: Endosperm Development

The moment triple fusion forms the primary endosperm nucleus (PEN), the construction of the

endosperm -- the nutritive tissue that will sustain the developing embryo -- begins. Endosperm

development characteristically starts before embryo development gets properly under way, ensuring

that by the time the zygote begins to divide in earnest, a food supply is already forming close by.

Flowering plants show three broadly recognised patterns of endosperm development, distinguished

chiefly by exactly when, relative to nuclear division, cell walls begin to form around the

descendants of the PEN.

The commonest pattern by far is the nuclear type. Here, the triploid PEN undergoes repeated

mitotic divisions, but these early divisions are NOT accompanied by cytokinesis (cell wall

formation); instead, the resulting nuclei simply accumulate together, free, within one shared mass

of endosperm cytoplasm, so that a single large endosperm cell may come to contain very many nuclei.

Cell wall formation is delayed, and when it does eventually begin, it typically starts at the

periphery of the endosperm and proceeds progressively inward, so that for some time an outer,

cellular region may surround an inner region that remains multinucleate and essentially liquid. The

familiar liquid 'water' found inside a tender (unripe) coconut, together with the solid white

kernel that lines the shell, is a classic and easily recognised example of nuclear-type endosperm:

the liquid represents the still free-nuclear, uncellularised centre, while the solid kernel

represents endosperm that has since cellularised at the periphery.

The second recognised pattern is the cellular type, in which cell wall formation accompanies every

single division of the PEN right from the very first one, so that the endosperm is fully cellular

from the outset and never passes through a distinct multinucleate, free-nuclear phase at all. This

pattern of development is characteristic of many members of the large plant family Asteraceae,

among others.

A third, less commonly encountered pattern, intermediate between the two, is the helobial type.

Here, the very first division of the PEN is markedly unequal, producing two chambers of quite

different sizes: a larger chamber toward the micropylar end of the embryo sac, and a smaller

chamber toward the chalazal end. Development then proceeds differently in the two chambers: the

larger, micropylar chamber typically develops in the free-nuclear manner characteristic of the

nuclear type described above, while the smaller, chalazal chamber divides more slowly, sometimes

remaining undivided, or dividing only a limited number of times. This pattern takes its name from, …