Tissues in Certain Monocotyledons. 37 
arrangement, such as is general in Monocotyledons, with a 
normal pith. In all the larger roots, however, the primary 
structure is much more complex, the pith being traversed by 
a variable number of xylem- and phloem-strands, generally 
associated together, and imbedded in groups of sclerenchyma. 
These peculiarities have been described so often that we need 
not dwell on them here. Strasburger has shown that the 
medullary strands of the root are directly continuous with 
vascular bundles of the stem 1 . Transverse sections through 
roots in which secondary growth has begun may show three 
different conditions : — 
(1) The cambium may have appeared in the pericycle, and 
the entire zone of thickening may be limited to the inside of 
the endodermis. This may still be the case in quite advanced 
stages, where there is a mass of secondary tissue, containing 
several concentric rows of bundles. 
(2) There may have been no pericyclic divisions at all. In 
these cases the primary structure of the cylinder is unaltered ; 
the secondary zone has been entirely superadded by the 
activity of a purely cortical cambium (see Fig. 5). 
(3) We may find a mixed condition, the secondary growth 
having begun in the pericycle, and then having been taken 
up by a cambium formed in the cortex. This is the case 
described and figured by Strasburger 2 . The two processes 
are mixed up in the most irregular way. In one radial row 
of cells the pericyclic divisions will have gone on for a long 
time before the cortical activity supervenes ; in the next row 
the pericyclic cambium ceases to act almost immediately, and 
nearly the whole growth is cortical. Hence we find fragments 
of the endodermis carried out into the secondary zone to a 
most variable distance (see Fig. 6). 
The same transverse section may show only one condition, 
or may include two, or all three, in different parts of the 
periphery of the cylinder. Thus in Fig. 4 conditions (2) and (3) 
are shown ; in Fig. 7, conditions (1) and (2), between which the 
1 Hist. Beitrage, III. p. 403. 
2 1 . c., Ill, p. 404, PI. V. Fig. 45. 
