62 
PLANT MORPHOLOGY. 
by attaching themselves to them or by crawling around 
them, when they are distinguished as twiners and climb- 
ers. Twiners ascend by a special circumnutating move- 
ment of the stem, as in the morning glory ; climbers, 
however, ascend by means of special structures, as the 
aerial roots of the ivy, and are termed root climbers ; 
or they may climb by means of leaves, as in the cle- 
matis, and these are known as leaf climbers; still others 
climb by means of tendrils, as in the grape, and these 
are known as tendril climbers. These tendrils, which 
are thread-like modifications of the stem, are in some 
cases provided with disk-like attachments for holding 
the plant in position, as in the Virginia creeper. 
Stems vary furthermore in size and form. While 
most stems are more or less cylindrical or terete, 
other forms also occur, as the flattened stems in the 
Cactaceas; triangular in the Juncacese, and quadrang- 
ular, in the Labiatae and Scrophulariacese. 
(6) THE STRUCTURE OF THE STEM. 
If we make a transverse section of a dicotyledonous stem 
a little below the growing point, we observe a differ- 
entiation of the tissues, as follows, this arrangement 
representing the primary structure: (1) The epidermis 
without root hairs; (2) a hypodermis; (3) a cortex; (4) 
an endodermis; (5) collateral fibrovascular bundles 
separated by primary medullary rays, and (6) a pith in 
the center. The essential difference between the primary 
structures of the stems of dicotyledons and monocoty- 
ledons, is that in the latter the fibrovascular bundle 
is of the concentric type, the medullary rays being 
wanting. 
On comparing the primary structure of the root with 
that of the stem, the following essential differences are 
observed : 
