THE ROOT. 
65 
which persist year after year are known as Perennials. 
Sometimes, as in temperate climates, the roots only are 
perennial, as in Phytolacca decandra, when the plant is 
spoken of as a perennial herb. 
(a) MODIFIED ROOTS. 
Roots which arise from the joints of the stem or other 
parts of the plant are known as secondary or adventi- 
tious roots. These include the aerial roots of the ban- 
yan tree, which are for the purpose of support; the 
roots of the ivy, which are both for support and climb- 
ing, and the roots of Indian corn, which serve both for 
support and the absorption of nourishment. Under 
this head may also be included the aerial roots of 
orchids and the root-like structures, called haustoria, 
of parasites (as of mistletoe), and saprophytes (as dod- 
der), which penetrate the tissues of their host plants. 
(6) THE STRUCTURE OF THE ROOT. 
Primary Structure. — If we make a transverse section 
of the root of a germinating plant, through the portion 
developing root hairs, the following arrangement of tis- 
sues is observed : (1) An epidermal layer with root hairs; 
(2j a hypodermis of a few layers of cells; (3) a cortex 
made up of a number of layers of parenchyma cells ; 
(4) a single layer of more or less lignified cells, known 
as the endodermis; (5) a central cylinder consisting of 
parenchyma and alternating groups of ducts and sieve, 
the number 1 of which varies and is more or less con- 
stant for certain genera. This arrangement consti- 
tutes what is known as the primary structure of the 
roots, and is essentially the same in both monocoty- 
ledons and dicotyledons. 
1 The terms monarch, diarch, triarch, tetrarch, polyarch, etc., are 
used to designate the number of plates or groups of ducts, there 
being usually a larger number of groups in the roots of mono- 
cotyledonous plants than in those of dicotyledons. 
