evolved from primitive plants with a 
reproductive biology much like this. 
The most compelling evidence for 
Sussex’s thesis, however, comes from 
the artificial culture of embryos. If 
a developing seed is carefully dis- 
sected under sterile conditions, its em- 
bryo can be surgically lifted out from 
the surrounding endosperm and ma- 
ternal tissue. Removed from the in- 
fluence of the mother, the embryo 
ceases storing proteins and starch in 
the cotyledons. When placed in the 
proper nutrient medium, it will sud- 
denly begin to grow and form a plant 
with roots and leaves. This has been 
called precocious germination, and it 
can occur at any time during the sec- 
ond phase of seed maturation. This 
second phase — with its cessation of 
cell division in the embryo, rapid stor- 
age of proteins, and subsequent de- 
velopmental arrest — is apparently a 
response to the influence of the ma- 
ternal environment. 
Closer inspection has revealed the 
agent of this maternal control. Sussex 
and his students monitored the flux 
of plant hormones in the developing 
embryo of the French bean seed. High 
concentrations of hormones known to 
stimulate cell division and expansion 
characterize early ontogeny; these are 
the same hormones found in the grow- 
ing plant after germination. At the 
critical transition point in the middle 
of embryogenesis, a new and quite 
different hormone, called abscissic 
acid, appears and rapidly increases in 
concentration. This substance is 
known to inhibit certain kinds of bio- 
chemical processes and is often as- 
sociated with drought-stressed plant 
tissue. The concentration of this hor- 
mone soon reaches a peak as the 
cotyledons become congested with nu- 
trients and the embryo undergoes de- 
hydration. Finally, the seed coat hard- 
ens, and the concentration gradually 
drops to a low level. 
Some scientists believe that the 
abscissic acid is synthesized in the 
maternal tissue and transported to the 
embryo through the endosperm. Sus- 
sex, however, feels that the abscissic 
acid may be created by the embryo 
itself in response to a more funda- 
mental signal from the mother, such 
as a gradual withholding of water. In 
either case, the certification of this 
hormone’s role comes again from ar- 
tificial culture. If an excised embryo 
is bathed in a nutrient medium sup- 
plemented with abscissic acid, preco- 
cious germination fails to occur. The 
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The "Evolutionary War'explained 
As National Geographic photojournalist John 
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the world's pre-eminent paleontologists, Donald 
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“Riveting and unsettling.. ..No one has laid 
out the conflicting theories of human 
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more comprehensively than Reader.” 
—Publishers Weekly 
MISSING LIN 
The Hunt 
John 
Reader 
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