ON THE POTATOE PLANT. O 



probably, we shall find that it is due to some very simple 

 law. The phenomena of endosmosis may explain many 

 of the facts which we observe. 



(20.) Various theories have been propounded with 

 respect to the cause of the motion of the sap, but a very 

 simple experiment will show how water may be taken from 

 the earth, and how the solid material may be sent back to 

 the root through the same vessel, and in fact how both 

 operations may proceed at one time. 



(21.) To explain this phenomenon, take a tube having a 

 rim at one end, and tie a piece of goldbeater's skin over 

 the rim. Into this tube put a solution of chloride of cal- 

 cium, sugar, or indeed of almost any body which will add 

 to the specific gravity of the fluid, and then immerse the 

 tube in a glass of pure water. After a short time the wa- 

 ter will pass into the tube, and at last run over like the 

 bleeding haulm. The leaf may be imitated by a piece of 

 blotting paper ; and it is possible exactly to regulate the 

 amount of attraction to the evaporation. The descent of 

 the salt or sugar, like that of the solid material descending 

 from the leaf, may be proved by examining the glass of 

 water, when it will be found that the solid matter has de- 

 scended, in spite of the powerful ascending current which 

 is opposed to the force of gravitation. 



(22.) To show the extent to which the potatoe draws 

 water from the earth, Marshall states, that, " on observing 

 the place where the haulm was cut, he found wet places 

 covered with vegetable blood, and that the stems bled pro- 

 fusely. On examining it more accurately he noticed that 

 each stem exhaled seven hundred and twenty drops in 

 twenty-four hours, and continued for about seven days, 

 giving off in drops two pints and a half of liquid in that 



