234 PART III. VEGETABLE PHYSIOLOGY. 



In cases of bleeding due to this cause, the sap-flow usually 

 ceases at night, or when the temperature is suddenly lowered 

 during the day, owing to the contraction of the gases in the tis- 

 sues of the stem. After the leaves unfold, the flow also ceases, 

 owing partly to the fact that with the season's advance, such sud- 

 den and considerable changes of temperature have ceased, partly 

 to the evaporation which now takes place from the leaves, and 

 partly to the avidity with which the protoplasm of the young 

 cells in the growing twigs and leaves absorb the liquids from 

 below. 



Practical Exercises. 



i. Take a wide- mouthed bottle of a half pint capacity or more, and fill it 

 half full of barley ; pour in a quantity of water barely sufficient to completely 

 cover the grain ; in the cork stopper bore a hole of such size that the tube of 

 a thermometer will nicely fit it ; insert the cork and thermometer in such a 

 manner that the bulb of the latter will be buried in the grain and the cork will 

 fit loosely in the neck of the bottle ; set the vessel away in a moderately warm 

 place for twenty-four hours and then observe. It will be found that the water 

 has been partly or wholly absorbed, and the grains have swollen so as to occupy 

 much more room in the vessel than at first. Moreover, on examining the ther- 

 mometer and comparing the temperature indicated by it with that of the out- 

 side air, it will be found that the heat has been developed in the process. 



Continue the experiment, keeping the grain moist but not submerged in 

 the water. The radicles will soon be observed to protrude through the ruptured 

 coats of the grains. Now compare the taste ot one of the grains with that of 

 one which has not been soaked. Both will have a farinaceous taste, but the 

 former will be perceptibly sweeter. Let the germination of the remainder pro- 

 ceed until the protruded radicles are fully two- thirds the length of the grain, 

 and then again compare the taste with that of an unsoaked grain. A more 

 decided difference will now be perceived, the sweet taste of the germinating 

 grain being still more evident, while the farinaceous taste has almost, if not 

 entirely, disappeared. 



Make a thin section of an unsoaked grain and treat it with a drop of iodine 

 solution, ft will immediately turn a dark violet color, showing that starch is 

 present in abundance. Test in the same way a section of a germinated grain 

 and if the germination has proceeded far enough, only a yellowish brown color 

 will be produced, showing that the starch has disappeared. It has been con- 

 verted into sugar as could readily be proved by applying Fehling's test. If at 

 this stage of germination the grain be poured out of the bottle and rapidly 

 dried at a temperature sufficiently high to destroy the vitality of the germ with- 

 out carbonizing the grain, say a temperature of about i6o c F., the result will be 

 malt, a substance largely used in medicine and in the manufacture of malted 

 liquors. 



