250 HOW CROPS GROW. 
exudation of it in drops upon the foliage. This may be 
noticed upon newly sprouted maize, or other cereal plants, 
where the water escapes from the leaves at their extreme 
tips, especially when the germination has proceeded under 
the most favorable conditions for rapid development. 
The bleeding of the vine, when severed in the spring- 
time, the abundant flow of sap from the sugar-maple, and 
the water-elm, are striking illustrations of this imbibition 
of water from the soil by the roots. These examples are, 
indeed, exceptional in degree, but not in kind. Hofmeister 
has shown that the bleeding of a severed stump is a gen- 
eral fact, and oceurs with all plants when the roots are 
active, when the soil can supply them abundantly with 
water, and when the tissues above the absorbent parts are 
full of this liquid. When it is otherwise, water may be 
absorbed from the gauge into the stem and large roots, un- 
til the conditions of activity are renewed. 
Of the external circumstances that influence the absorp- 
tive power of the root, may be noticed that of tempera- 
ture. By observing a gauge attached to the stump of 
a plant during a clear summer day, it will be usually no- 
ticed that the mercury begins to rise in the morning as 
the sun warms the soil, and continues to ascend for a num- 
ber of hours, but falls again as the sun declines. Sachs 
found in some of his experiments that at a temperature of 
41° F,; absorption, in case of tobacco and squash plants, 
was nearly or entirely suppressed, but was at once renewed. 
by plunging the pot into warm water. 
The external supplies of water,—in case a plant is sta- 
tioned in the soil, the degree of moisture contained in this 
medium,—obviously must influence, not perhaps the im- 
bibing force, but its manifestation. 
The Rate of Absorption is subject to changes depend- 
ent on other causes not well understood. Sachs observed 
that the amount of liquid which issued from potato stalks 
