
THE STEM 115 
THE TWO TYPES OF STEMS 
Flowering plants, as previously noted, are divided into two gen- 
eral classes, monocotyledons and dicotyledons. Monocotyledons are 
plants that have one cotyledon, or seed leaf, and dicotyledons those 
that have two cotyledons. The cotyledons are the first leaves pro- 
duced by plants and are usually found in the seed. When there are 
two, they are opposite each other, but plants with two seed leaves 
frequently do not have their subsequent leaves opposite each other. 
The leaves of dicotyledons have netted veins (Fig. 16), while 
those of monocotyledons usually have parallel veins (Fig. 16). 
If a dicotyledon reaches any considerable size, it has a bark 
which can be readily stripped from the remainder of the stem, 
while monocotyledons do not have bark. Stems of monocotyle- 
dons reach a certain diameter and then do not usually become 
thicker, while stems of dicotyledons generally continue to grow 
in thickness as long as the plant lives. The floral parts of mono- 
cotyledonous plants are usually arranged in threes or in multi- 
ples of three, while in dicotyledonous plants the numbers vary, 
but they rarely occur in threes or multiples of three. The most 
important monocotyledons are grasses, including bamboos, sugar 
cane, and all cereals such as corn, rice, and wheat; sedges (plants 
which resemble the grasses in appearance); palms (Fig. 108); 
members of the lily family and related families; orchids; aroids; 
and members of the ginger (Fig. 172) and banana (Fig. 66) 
families. Dicotyledonous plants are much more numerous and 
include all ordinary trees, shrubs, and many small plants. The 
structure of the stem of a monocotyledonous plant is, as we 
shall see, very different from that of a dicotyledonous plant. 
ANATOMY OF DICOTYLEDONOUS STEMS 
General regions. In young dicotyledonous stems there are 
three distinct regions (Fig. 113). The outermost is the epider- 
mis. Within the epidermis there is a zone that has the shape 
of a hollow cylinder and is known as the cortez. The center 
of the stem is occupied by a solid column, the stele. 
