CH. V] WOOD. 67 



and this tissue helps us to make out the general structure 

 of a tree-trunk : for though it looks small in comparison 

 with the diameter of the stem, it is the same pith that 

 looked big in the section of the plumule. The pith 

 has not grown, and the mass of new tissue has therefore 

 nothing to do with it. It ought to be possible to 

 discover another fixed point by which to guide ourselves. 

 The cambium ring should still be recognizable, since 

 it remains perpetually young, and therefore unchanged. 

 Between the bark and the wood there is found a layer 

 of cells (noticeable in the spring-time for its sliminess) 

 which proves under the microscope to be the cambium, 

 the direct descendant of the cambium ring of the seedling 

 and like it composed of delicate meristematic tissue. 



With the help of these two fixed points, the pith and 

 the cambium, the tissues of the oak branch may be classi- 



FIG. 29. 



TRANSVERSE SECTION OF AN OAK-TRUNK, 25 years old. 

 From Le Maout and Decaisne. 



fied. What lies between the pith and the cambium, and 

 is known as wood, must be xylem ; all outside the cambium 

 must be phloem and cortex. Again, there were in the 



52 



