velopmental pathway of cell division 
and tissue differentiation. In most 
higher plants, however, the mother im- 
poses a detour on the offspring that 
channels it out of growth and into 
a phase of dormancy and desiccation. 
There it must await a signal to resume 
growth. Germination, then, represents 
a return to the original developmental 
pathway. 
Sussex divides embryogenesis — the 
formation of the embryo within the 
seed — into two phases. Initially there 
is a period of intense cell division, 
with relatively little increase in size, 
as the fundamental structure of the 
future plant is roughed out. During 
this time, endosperm tissue grows rap- 
idly on maternal resources and sur- 
rounds the offspring. The second 
phase commences suddenly about 
halfway through embryogenesis. The 
water content of the embryo begins 
to fall. Cell division ceases completely, 
and the embryonic body proper stops 
development. The offspring starts to 
digest the endosperm, and its cotyle- 
dons, now rapidly expanding, begin 
to store the newly formed proteins and 
starch. Finally, the seed coat dries and 
hardens into a constraining, imperme- 
able jacket. The imprisoned embryo 
must survive on its own reserves until 
released by hydration. 
Germination reverses this process. 
The seed absorbs water rapidly, ex- 
panding its cells and cracking the en- 
casing coat. The rudimentary root pen- 
etrates the gap, and cell division 
accelerates throughout the body of the 
embryo. The remaining food stores in 
the cotyledons begin to disintegrate 
and pass to the growing tips. Incipient 
leaves soon form, shedding the ma- 
ternal covering and expanding into the 
light. 
Does the mother plant really impose 
developmental arrest on her young 
during embryogenesis, as this view 
suggests? Sussex calls upon three 
kinds of evidence to sustain his in- 
terpretation. First, there are vivipa- 
rous species, whose seeds germinate 
while still attached to the mother 
plant. The growing embryos of coastal 
mangroves, for instance, never enter 
the dormant phase of seed formation, 
germinating well before they drop into 
the tide to root in local mudflats. 
A second kind of evidence comes 
from the biology of primitive vascular 
plants, which may roughly resemble 
the ancestors of modern seed plants. 
Like mangroves, the embryos of ferns 
do not require dormancy for successful 
development. Their entire reproduc- 
tive biology, however, is interestingly 
different: ferns reproduce by spores 
rather than by seeds and the offspring 
does not actually grow on the body 
of the mother plant. Instead, a spore, 
carrying half the mother’s genes, dis- 
perses on the wind and begins to grow 
some distance from the original stalk. 
Slowly, a rudimentary, heart-shaped 
plantlet forms. This plantlet may en- 
gage in an intimate nutritional asso- 
ciation with an underground fungus 
or develop a green disk of photosyn- 
thesizing cells. When moisture be- 
comes abundant, the plantlet forms 
an egg nucleus that also has half the 
mother’s genes. An identical process 
leads to the production of sperm, or 
gametes. For fertilization to occur, 
these male gametes must swim 
through a surface film of water to 
a nearby plantlet. Sex is thus 
obligately associated with a wet habi- 
tat. The embryo commences cell di- 
vision, differentiates leaves and roots, 
and completely crushes the small 
plantlet in the process of growth. Nei- 
ther desiccation nor dormancy has any 
necessary part in this pattern of plant 
formation. The seed habit may have 
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