110 FIELD, FOREST AND FARM 



the outermost layers, or those of most recent forma- 

 tion. Repeated experiment leaves no doubt on this 

 point. When a tree is cut down at the time of the 

 sap's greatest' activity, we find the sap-wood moist 

 and the older wood perfectly dry. Finally, in herba- 

 ceous plants the sap ascends through the whole body 

 of the stem. Suspended during the winter on ac- 

 count of the absence of foliage, this ascent of the sap 

 becomes remarkably brisk at the awakening of vege- 

 tation. Then it is that fruit-trees shed tears, so 

 to speak, where the pruning-hook has left its mark ; 

 or, in other words, the ascending sap oozes from the 

 openings of the severed ducts. These tears are espe- 

 cially noticeable in the grape-vine, where it has re- 

 cently been trimmed. 



"Now what would you expect to find in this liquid 

 if you collected some of it as it trickles in the form 

 of tears either from the vine or from a fruit-tree? 

 Many things, doubtless, you will say, since this pre- 

 cious liquid is the prime source of all that the plant 

 contains in itself. If such is your thought, unde- 

 ceive yourselves: ascending sap is little more than 

 clear water, and often it is very difficult for science 

 to prove beyond a doubt the presence in it of vari- 

 ous substances in solution, so minute a fraction of 

 the whole do they compose. Among these substances 

 the most frequent are compounds of potash, of lime, 

 of carbonic acid gas, traces of phosphates, and com- 

 pounds of nitrogen or ammonia. In short, the liquid 

 from which the plant is to derive its nourishment is 

 the weakest sort of broth, composed of an enormous 



