Subclass Dicotyledones 



'Lx\NTS of this subclass have a more complex stem structure than the 

 Monocotyledones, their fibrovascular system being arranged in con- 

 centric layers, which are divided into wedge-shaped segments by 

 cellular tissue, called medullar)^ rays, radiating from a central column 

 of similar material, called the pith. The growth of these stems takes place in the 

 cambium, a zone of soft tissue between the wood and bark, by which the stem is 

 enlarged, the annual suspension of growth caused by a decrease of temperature 

 or lack of moisture, producing the concentric bands usually called annual rings, 

 which in our climate can generally be depended upon as representing one year's 

 growth. They are often called Exogens or outside growers. 



The leaves are netted- veined. They often have stipules and are frequently 

 compound, and their position upon the stem may be alternate, opposite, or verti- 

 cillate. The flowers are various in structure and often more complex than those 

 of most ISIonocotyledones. The embr\'o has 2 seed-leaves. 



This subclass contains by far the greater number of our deciduous-leaved 

 trees, and all of the so-called hard woods are derived from them; it is composed 

 of two series of plants, (i) Choripetalae, in which the corolla of the flower is either 

 wanting, or the petals are separate from each other, and (2) Gamopetalas, in which 

 the petals are more or less united, forming a saucer-shaped, um-shapcd, bell- 

 shaped, or more or less tubular corolla; an interesting exception to this structure 

 is found in the Ashes (Fraxinus) trees of the Olive Family, in many of which the 

 flowers have no corolla, but other points of structure cause them to be grouped 

 as Gamopetalae. 



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