250 HOW CROPS GEOW. 



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 occurs 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 



