SPERMATOPHYTES 239 
flowers of monocotyledons have their members in sets of three; while 
the flowers of dicotyledons have their members in sets of five or four. 
These characters are by no means of equal value, the character of the 
embryo being the only one without serious exception. There are mono- 
cotyledons with vascular cylinders, with open venation, and with flowers 
not in threes ; and there are dicotyledons with scattered vascular bundles, 
with closed venation, and with flowers in threes. It is not so much a 
single character, therefore, that distinguishes a monocotyledon from a 
dicotyledon, as a combination of characters. 
There are recognized also two great divisions of dicotyledons, the 
Archichlamydeae and the Sympetalae, and the conspicuous character 
which distinguishes them is implied in their names. The Archi- 
chlamydeae have either no petals or petals entirely separate from one 
another (free), and this is recognized as the primitive condition of the 
perianth (chlamys); while in the Sympetalae the petals develop so as 
to form tubes of various kinds. This distinction is superficial and 
breaks down in certain cases, but it holds generally and is convenient. 
These three great groups of angiosperms are related to one another 
as follows: the Archichlamydeae are recognized as including the most 
primitive angiosperms; from the more primitive Archichlamydeae 
the monocotyledons probably have arisen as a special branch; while 
from the more advanced Archichlamydeae the Sympetalae have arisen 
and are clearly the highest group of angiosperms. It will be necessary 
to keep in mind these three groups and their relationships in order to 
understand the following discussion. 
STEM 
A description of the general structure of a vascular stem has been 
deferred to angiosperms, whose stem may be taken as an illustration 
of the general features of all vascular stems. 
Elongation. The tip of the stem is the growing point, consisting 
of a group of very actively dividing (meristematic') cells. Among 
pteridophytes this group of meristematic cells is usually represented 
by a single apical cell. All the tissues of the stem are derived from the 
cells of the growing point, whose activity very soon results in the 
appearance of three more or less distinct generative regions : (i) der- 
matogen, which later forms the epidermis,' (2) periblem, which later 
forms the cortex; and (3) plerome, a central cylinder which forms the 
stele, in which the vascular elements appear (fig. 538). 
