70 OAK. [CH. V 



assimilating organs, but it is not clear why the vessels 

 formed in the spring should be bigger than the later 

 formed ones. The most probable explanation is that of 

 Strasburger namely, that the large vessels are needed for 

 the rise of sap in the trunk, which occurs in the spring. 



FIG. 31. 



DIAGRAM ILLUSTRATING TWO TYPES OF LONGITUDINAL SECTION. 

 The line R would divide the cylinder by a radial section, T by a 

 tangential section. 



Certain points can be made out by means of longitu- 

 dinal sections examined with a simple lens or with a low 

 power of the microscope. Longitudinal sections are of 

 two kinds. If a branch is divided longitudinally by an 

 incision which passes through the centre, the surface 

 exposed is a radial longitudinal section ; this is shown in 

 fig. 31, where R represents the line along which the 

 branch (here seen in section) is divided. 



But the incision may be longitudinal, that is to say, 

 parallel to the axis of the branch, and yet may not pass 

 through the centre, it is then called a tangential section, 

 as shown by the line T in fig. 31. 



If an oak branch of three or four years old is split down 

 the middle, and if the radial section thus exposed is 



