The Angiosperms 159 



MONOCOTYLEDONS AND DICOTYLEDONS 



A survey of the multitude of angiosperms which 

 everywhere abound shows two pretty well-marked 

 series of forms. One series may be represented by 

 a tulip or daffodil, herbaceous plants having smooth, 

 elongated leaves with inconspicuous, unbranched, 

 parallel veins, and flowers with the parts in sets of 

 three, the two outer sets of leaves being alike. If 

 the seedling plant is examined, it will be found to 

 possess a single seed-leaf or " cotyledon " ; hence 

 these plants are called " Monocotyledons." 



Mustard may be taken as a type of the second 

 series. The seedling shows two opposite cotyledons ; 

 hence the name " Dicotyledon " applied to the series. 

 The young plant increases rapidly in size, and an 

 examination of the stem structure shows that the 

 woody bundles of the stem continue to increase in 

 size by the addition of new tissue derived from a 

 layer of permanently growing tissue, or cambium, 

 which results in a secondary thickening of the 

 stem. This in the woody-stemmed shrubs and trees 

 exhibits annual growth rings like those in the trunk 

 of a pine or other coniferous tree. No monocoty- 

 ledons show this type of secondary growth in thick- 

 ness. 



The leaves in the mustard are more or less deeply 

 divided or cut, and their veins are much branched 

 and more or less united into an elaborate network. 

 The flowers have the parts in fours, although the 



