CHAPTER XI 
DICOTYLEDONS 
Tue Dicotyledons, the second great division of the 
Angiosperms, comprise the major part of existing plant 
forms, and it is among these that the vegetable organism 
reaches its most complete expression. Compared with 
the Monocotyledons they are both more numerous and 
more varied. With the exception of the grasses and a 
few aquatic types, the Monocotyledons are seldom abun- 
dant enough, at least in temperate regions, to give a 
prevailing character to the vegetation of any district ; 
the Dicotyledons, on the other hand, are often gregari- 
ous and better able to hold their own in the struggle 
for existence. All the forest trees of temperate regions, 
except Conifers, are Dicotyledons, and except for the 
grasses, hardly any of the aggressive plants we call 
weeds are Monocotyledons, and, as we have seen, very 
few types of the Monocotyledons attain the size of trees. 
The most constant character shown by the Dicotyle- 
dons is the presence of two cotyledons or primary 
leaves in the embryo (Fig. 48). A few cases where 
only a single cotyledon is present can usually be 
accounted for by the abortion of one of the cotyledons, 
but it is possible that there may be forms which are 
intermediate in this respect between the two great 
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