32 THE OAK 



distinct outlines ; these alternate groups are known as 

 phloem, and we may shortly examine the elements of 

 which they are composed, as before, by comparing 

 sections of various kinds. 



Here, again, we find the chief structures in the phloem 

 are also vessels i.e. long, tubular organs but very dif- 

 ferent in detail from the vessels of the xylem. 



In the first place, their walls are thin and soft, and 

 composed of the unaltered cellulose which is so charac- 

 teristic of young cells (instead of being hard like the 

 Hgnified walls of the xylem vessels) ; then, again, they 

 contain protoplasm and other organised cell contents, 

 instead of merely air and water. Finally, they are not 

 so completely tubular as the typical xylem vessels are, 

 because the transverse septa of the constituent cells are 

 not absorbed, but are merely pierced by fine strands of 

 protoplasm, and therefore look like sieves when viewed 

 from above whence the name ' sieve-tubes.' In the 

 phloem also we find cells phloem-cells packed in 

 between the sieve-tubes. 



If we shortly summarise the above we find that the 

 root consists of an axis-cylinder surrounded by a cortex 

 and the piliferous layer. At the tip the whole is 

 covered by the root-cap, which is organically connected 

 with the embryonic tissue which forms all these struc- 

 tures. The axis-cylinder is somewhat complex ; it is 

 sheathed by the endodermis and the pericycle, the 

 latter of which gives origin to the new rootlets. Inside 

 the pericycle are the vascular bundles running up and 



