58 
PLANT MORPHOLOGY. 
The following diagram of the secondary structure of 
a dicotyledonous root may be of assistance in under- 
standing the origin and relation of the tissues com- 
prising it : 
' Pith, which may be wanting. 
Wood made up of -j 
l 
Xylem 
/ 
Cambium produces 
L 
Composed of ducts, wood paren- 
chyma and wood fibers ; or tra- 
cheids may replace these, or be 
associated with them. These are 
arranged in groups forming radial 
rows which are separated by med- 
ullary rays. 
' Phloem 
Bark made up of ■ 
f Consisting of sieve and bast paren- 
chyma; bast fibers may also be 
-j present These are arranged in 
groups and form radial rows which 
[ are separated by medullary rays. 
Phellogen, producing parenchyma and cork. 
The root branches arise as the product of a meristem, 
known as the pericambium, situated’ inside of the endo- 
dermis. The tissues forming the branches are directly 
connected with the fibrovascular tissues of the root 
and protrude through the overlying tissues without 
having any connection with them. The structure of 
the branches thus formed corresponds to the primary 
structure of the roots, and in the case of dicotyledonous 
roots may also subsequently develop a secondary 
structure. 
Contraction of the roots is observed in both mono- 
cotyledons and dicotjdedons, it being most apparent in 
the former, as in veratrum viride. The uneven or cork- 
screw-like appearance is due to this contraction, which 
arises as follows : Some of the longitudinally elongated 
cells beneath the epidermis absorb large quantities 
of water, which causes them to assume a spherical 
form (as the cells of a potato are altered on boiling^ 
