262 The Sap. 



The Sap, 



MODIFIED FROM J.INDLEY. 



Foil the sustenance of plants a fluid is necessary, which is 

 absorbed by the roots from the earth, then sent upwards into the 

 stem, afterwards impelled into the leaves, whence it descends 

 through the liber, transferring itself to the inmost parts of the wood. 

 This fluid, which constitutes the blood of plants, is called the 

 sap. When first introduced into the system, and even when 

 altered in some degree by having dissolved the various sub- 

 stances it encounters in its passages, it is true sap ; afterwards, 

 when its nature has become more changed by elaboration in the 

 leaves, it becomes what is called the proper juice. If the sap 

 be examined in its most simple state, it will be found to consist 

 of water, mucilage, and sugar. As the two last cannot have been 

 absorbed directly from the earth, they afford a good proof of the 

 power of the minute spongioles or terminal extremities of the 

 roots, 10 act like stomachs and digest it. In addition to the sup- 

 ply of sap which is obtained by the roots, a certain quantity is 

 no doubt also absorbed from the atmosphere by the leaves, as is 

 evident from succulent plants, which will continue to grow and 

 acquire weight long after the roots are severed from the earth. 

 This absorption on the part of the leaves generally takes place 

 at night, or in cloudy weather ; while perspiration, on the other 

 hand, goes on in the day time, in bright weather. Link says, 



THAT THE FOOD OF PLANTS MUST BE COMPOSED OF OXYGEN, HY- 

 DROGEN, carbon and nitrogen. Water, consisting of oxygen 

 and hydrogen alone, is not sufficient. Many experiments in- 

 deed have been instituted to prove that pure water is a sufficient 

 food, especially by VanHelmont, Eller, Bonnet, DuHamel, and 

 others ; but it is probable, as Wallerins has inferred, that the 

 water out of which plants are formed already contains the 

 necessary principles. To this it is objected that plants grown 

 in water alone never arrive at perfection, or mature their seeds. 

 But this is not strictly true ; they do perfect their seeds ; but it is 

 not surprising that crude water should be insufficient for pur- 



