28 rOISONOUS PLANTS 



or edge. They are represented in the tiny embryo 

 of the Buttercup (Figs, i ; 9, 10 ; p. 17). When 

 such seeds germinate, the two cotyledons often 

 come above ground and turn green, as in mustard 

 and cress. 



In germinating onions, a single leaf comes up at 

 first. This is the one cotyledon, as they belong 

 to the Lily family, which is a monocotyledonous 

 group. 



These two Classes, are, however, known by a 

 collection of characters, and not only by their 

 embryos. 



Thus, when a seed germinates, as of a carrot or 

 parsnip, it develops an axial or tap-root, if it be a 

 dicotyledon. In monocotyledons this is always 

 arrested and secondary roots issue from the stem. 



Again, the stem of a dicotyledon, if woody, has 

 concentric cylinders of wood, one being formed 

 every year ; so they appear as rings in a transverse 

 section of any of our timber-trees or shrubs, which 

 thus reveal the age of the tree. 



In a monocotyledon the wood is broken up into 

 isolated cords running anywhere through a mass 

 of soft tissue. Such is easily observed in a stick 

 of asparagus. 



The leaves of all our trees and shrubs have 

 a reticulated " venation " ; i. e. the woody frame- 

 work upon which the green tissue is spread out 

 forms a sort of network. 



