272 HOW CROPS GROW. 



Of the external circumstances that affect this absorp- 

 tive power, heat and light would appear to be influential. 

 By observing a gauge attached to the stump of a plant 

 during a clear summer day, it will be usually noticed 

 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, in case of potted 

 tobacco and squash plants, absorption was nearly or 

 entirely suppressed by cooling the roots to 41° F., but 

 was at once renewed by plunging the pots into warm 

 water. 



The external supplies of water, — in case a plant is 

 stationed in the soil, the degree of moisture contained in 

 this medium, — obviously must influence any manifesta- 

 tion of the imbibing force. But full investigation shows 

 that this regular daily fluctuation is a habit of the plant 

 which is independent of small changes of temperature, 

 and even of considerable variation in the amount-of mois- 

 ture of the soil. 



The rate of absorption^ is subject to changes depend- 

 ent on causes not well understood. Sachs observed 

 that the amount of liquid which issued from potato 

 stalks cut ofi just above the ground underwent great 

 and continual variation from hour to hour (during rainy 

 weather) when the soil was saturated with water and 

 when the thermometer indicated a constant temperature. 

 Hofmeister states that the formation of new roots and 

 buds on the stump is accompanied by a sinking of the 

 water in the pressure-gauge. 



Absorption of Nutriment from the Soil. — The food of 

 the plant, so far as it is derived from the soil, enters it 

 in a state of solution, and is absorbed with the water 

 which is taken up by the rootlets. The absorption of 

 the matters dissolved in water is in some degree inde- 

 pendent of the absorption of the water itself, the plant 



