cotyledons proceed to swell, storing 
nutrients at a rate proportional to the 
concentration of abscissic acid, and 
the embryo becomes metabolically 
quiescent. The maternal influence has 
been simulated. 
Abscissic acid, then, appears to be 
the chemical mediator of the devel- 
opmental arrest imposed by the 
mother. It signals the impending des- 
iccation of the offspring and initiates 
preparation for survival. It is a har-. 
binger of stress, and it will serve a 
related function in the shoots of the 
adult. We know that its concentration 
increases in the leaves of drought- 
stressed individuals as the effects of 
declining water are felt. Ordinarily, 
water moves from the soil into the 
roots and through the vascular system 
of the plant to the surface of leaves, 
where it evaporates through small ap- 
ertures called stomata. This liquid 
flow flushes each plant cell and pro- 
vides the pressure that gives rigidity 
to stems and leaves. If soil moisture 
begins to decline at the roots, the low- 
ered flow of tissue fluid leads to wilt- 
ing leaves. At these times of declining 
water, however, the concentration of 
abscissic acid in the vicinity of the 
stomata increases, causing the sur- 
rounding cells to expand and close 
off the escape of fluid. With its in- 
ternal pressure held constant, the plant 
can remain turgid while awaiting the 
relief of drought. 
How remarkable that the embryo 
should respond in its own style to this 
messenger of stress. Yet this is nicely 
coincident with recent thinking on the 
origin of the flowering plants, in which 
embryo dormancy has undergone its 
most elaborate evolution. The great 
evolutionary biologist G. Ledyard 
Stebbins believes that the earliest 
flowering plants probably appeared 
more than 160 million years ago. Their 
origin is usually assumed to have been 
in the humid tropics, where there is 
often little seasonal variation in cli- 
mate. Stebbins, however, argues that 
subtropical montane habitats, subject 
to seasonal drought, are a more likely 
cradle for flowering-plant evolution. 
Seasonality would greatly constrict 
the period of time permitting repro- 
duction and would expose embryonic 
offspring to unfavorable conditions of 
drought. To survive, the embryo would 
require protection from imminent des- 
iccation. Would not any individual 
able to respond to maternal signals 
of drought stress be more likely to 
survive to the next generation? The 
embryo’s ability to store food and be- 
come dormant did indeed prove highly 
adaptive. This capacity also provided 
the mother plant with an incipient 
mechanism to control her growing 
young, which may have become es- 
sential for her own survival. 
Indulge me, for a moment, in a 
bit of historical speculation on the evo- 
lution of the seed habit in the ancestors 
of modern flowering plants. Ferns, you 
may recall, are totally dependent upon 
free-flowing water for sexual repro- 
duction. The male sperm must swim 
to the female egg that has formed 
in the small, rudimentary plantlet pro- 
duced by a spore from the mother 
shoot. Such a system could not survive 
in conditions of even minimal drought; 
the plantlet alone would shrivel and 
die. The fertilization of the egg within 
maternal tissue solved this problem. 
Rather than releasing spores to the 
wind, the mother plant retained this 
stage within her own body, protecting 
it from desiccation. As a result, any 
seasonal declines in moisture no longer 
restricted reproduction. 
But bringing the developing embryo 
into the safety of the maternal en- 
vironment was not without its com- 
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