192 ELEMENTS OF PLANT ANATOMY. 
corresponding gréup of phloem. It may, however, be quite as 
logical to regard the whole cylinder as a single central bundle, 
and those who prefer this explanation give to it the names 
monarch, diarch, triarch, polyarch, etc., according as there are 
one, two, three or more xylem groups.. In the root of most vas- 
cular cryptogams there are two or more such groups. (Fig. 52.) 
The root of monocotyledons differs but little from that of 
the cryptogams in respect to its vascular system. In some plants 
there is a well developed pith occupying the central portion of 
the cylinder, the whole being surrounded by the endodermis or 
sheath. Some few monocotyledons, for example Pandanus and 
a few palms, have roots in which the bundles are separate. The 
ground and epidermal systems are also similar to those of the 
cryptogams except that the hypodermal sheath of the crypto- 
gamic root is generally parenchymatic, while in the mono- 
cotyledons it is more frequently a regularly developed endo- 
dermis.1 
In the early stage of their development the roots of dicoty- 
ledons resemble those described above in all important features. 
In case of many annuals this construction remains during 
the entire life of the plant, and of these it may be said, the 
structure of their root is the same as that of the monocotyle- 
dons. It is, however, very different with other annuals, with 
perennials, and all woody or long-lived plants. In these there 
is a later development corresponding to that before referred to 
as secondary growth of stem, but it differs from this in that it 
begins much earlier in the life of the plant. The complicated 
structure of the root tip and its various methods of growth 
render the developmental history difficult to follow in all its 
1 The name endodermis has been given to the cylinder or sheath surround- 
ing a bundle or collection of bundles. This sheath consists of a single layer of 
cells derived from the ground system ; they are longer in the axial than in the 
other two directions, usually contain starch and their walls are frequently suber- 
ized. 
