xx HISTOLOGY OF TANGLES 205 



and a tree or shrub is quite obvious : when cut across they 

 are seen to consist of a nearly homogeneous substance of 

 the consistency of soft gristle, neither bark, wood, nor pith 

 being distinguishable. Under the microscope, however, 

 the cells of which they are composed are seen to vary 

 considerably in form and size, some of them even assuming 

 the characters of what we shall learn in our studies of the 

 higher plants (Lesson XXIX) to distinguish as sieve-tubes. 



