CHAPTER II. 



LEADING PRINCIPLES IN THE GROWTH OF TREES CIRCULATION 

 OF THE SAP. 



DURING the growth of a tree, a most interesting process is 

 going on, which should be well understood by every one 

 engaged in cultivation. The sap enters from the soil into 

 the spongioles, or the minute spongy extremities of the 

 finely-branched fibres ;* it passes up these fibres or fine roots, 

 through the thousands of minute tubes or sap vessels, (which 

 are minuter than the smallest hair,) until united into the 

 larger roots ; the union of these little currents of sap some- 

 what resembling that of the innumerable rills which con- 

 stitute a large river. On reaching the trunk or stem, it 

 flows upward through the myriads of little vessels in the 

 alburnum or sap-wood, and reaching the branches, becomes 



again subdivided through them, 

 and is sent out into all the ex- 

 tremities of the smallest shoots. 

 A young apple tree an inch in 

 diameter, consists of about one 

 million of these little sap-tubes 

 united together, and a single one- 

 year shoot contains more than ten 

 thousand. The annexed figure 

 represents a greatly magnified 

 cross-section of a small portion of 

 a peach-shoot, showing the sap 

 vessels. 



Passing up the leaf-stalks from 

 the shoots, the sap emerges for the 

 Cross-section of the sap- vessels of a first time to the light, through the 



innumerable microscopic veins 



* For a magnified representation of a spongelet. see p. 31. 



