430 Ganong . — The Comparative Morphology of 
last trace of the albumen in combination with an early spread 
of the cotyledons to the light. 
As the lower end of the hypocotyl issues from the seed, 
its positive geotropism manifests itself, and it bends down- 
wards ; at the same time from the swollen collar there grows 
out at about 45 0 a great number of long slender straight 
hairs, which attach themselves to the soil or other substratum, 
forming a series of cords binding the young plant firmly to 
the soil (see Fig. 1 J c, d). It seems to me that these hairs 
come out at a constant angle to the stem, and are not 
geotropic, in which case it is an interesting question how 
they are guided in assuming that direction. This method 
of binding the seedling to the soil is well known elsewhere 
(see Klebs), and is generally considered necessary to hold 
the seedling in position until its root becomes fixed. But 
it, like other methods of fastening young germinating 
embryos to the ground, may have rather the significance of 
providing a resistance for the root to work against in the 
mechanical work of forcing its way into the ground, as 
otherwise the embryo would simply be lifted, and there would 
be no leverage for the work. The root at first grows very 
slowly, but the hypocotyl, as it issues from the seed-coat, 
begins at once to swell at the base, and this swelling extends 
regularly upward as it issues from the seed. Finally, only 
the cotyledons, sometimes three in number, are left, and 
these are still very small ; but now either these are disengaged 
and swell also, or else they remain, one or both, for a time 
in the seed ; sometimes however they seem unable to with- 
draw themselves from the seed-coat, and they are held and 
cut by the seed-coat as they swell, though in their more 
vigorous growth at home in the deserts this probably does 
not happen. As soon as the hypocotyl is well out of the 
coat, its upper end manifests its apogeotropism and curves 
upwards. The embryo, on issuing from the seed, turns first 
pale green ; but very soon, in many species, red colour appears. 
At first the cotyledons are tightly pressed against one another, 
but they separate later, and, as the epicotyl begins to grow, 
