CHAPTER XI 



DICOTYLEDONS 



THE 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 



199 



