240 HOW CROPS GROW. 
strated by placing the old, brown-colored roots of a plant 
in water, but keeping the delicate and unindurated ex- 
tremities above the liquid. Thus situated, the plant with- 
ers nearly as soon as if its root-surface were all exposed to 
the air. 
b. Its Rapid Extension in Length, and the vast Sur- 
face which it puts in contact with the soil, further adapts 
the root to the work of collecting food. The length of 
roots in a direct line from the point of their origin is not, in- 
deed, a criterion by which to judge of the efficiency where- 
with the plant to which they belong is nourished; for 
two plants may be equally flourishing—be equally fed by 
their roots—when these organs, in one case, reach but one 
foot, and in the other extend two feet from the stem to 
which they are attached. In one case, the roots would be 
fewer and longer; in the other, shorter and more numer- 
ous. Their aggregate length, or, more correctly, the ag- 
eregate absorbing surface, would be nearly the same in 
both. 
The Medium in which Roots Grow has a great influence 
on their extension, When they are situated in concen- 
trated solutions, or in a very fertile soil, they are short, 
and numerously branched. Where their food is sparse, 
they are attenuated, and bear a comparatively small num- 
ber of rootlets. Illustrations of the former condition are 
often seen. Bones and masses of manure are not infre- 
quently found, completely covered and penetrated by a 
fleece of stout roots. On the other hand, tbe roots which 
grow in poor, sandy soils, are very long and slender. 
Nobbe has described some experiments which com« 
pletely establish the point under notice. (Vs. St, IV, p. 
212.) He allowed maize to grow in a poor clay soil, con- 
tained in glass cylinders, each vessel having in it @ quan- 
tity of a fertilizing mixture disposed in some peculiar man- 
ner for the purpose of observing its influence on the roots. 
When the plants had been nearly four months in growth, 
