86 FAMILIAR LESSONS IN BOTANY. 



220. The experiment made so often by school-boys on 

 mustang grape-vines will give you some idea of the quan- 

 tity of sap and the power with which it is propelled up- 

 ward. A branch is chosen, say an inch in diameter, and 

 cut off four feet from the ground ; over this the neck of a 

 bladder is stretched and secured firmly by a rosined thread. 

 In a few hours this bladder will be full of sap, and soon it 

 will burst into shreds with an explosion like that of a 

 pistol. 



227. The force with which sap ascends has been estimated 

 to be five times greater than that which impels the blood 

 in the great crural artery of the horse. After rising to the 

 leaves, exhaling through them superfluous moisture, ex- 

 changing gaseous compounds, yielding up some, receiving 

 others, it commences its downward motion, and is now true 

 sap, containing all the peculiar products of the plant, gums, 

 resins, oil, milk, and mucus. 



228. Its principal flow is now through the bark, though it 

 spreads horizontally also. It makes a deposit of tissue for 

 a new layer of wood, and continues downward, hardening the 

 root by deposits of various matter. It adds fresh cellular 

 tissue to the extremities to form new spongioles, convert- 

 ing them into epidermis. 



229. We thus see each year a new layer of wood and bark 

 deposited; the soft wood of the year previous becoming 

 harder and firmer from tffe deposit of sclerogen and other 

 substances left by the descending sap. Some approxima- 

 tion may be made to the age of a tree by counting the 

 number of rings the yearly sap deposits in a transverse 

 section of its trunk. 



226. What of. the grape-vine experiment ? 



227. What is the ascending force computed to be equal to? Describe its ac- 

 tion after it reaches the leaves. 



228. Where is the principal flow of the sap after it reaches the leaf? Is it 

 confined entirely to the bark ? Describe its farther downward flow. 



229. How does soft wood become hard? How can you judge the age of a 

 tree? 



