LILIACEOUS PLANTS. 61 



81. Besides the characters just mentioned, there is still 

 another one of great importance which Dicotyledon* 

 possess in common. It is the manner of growth of the 

 stem.. In the Willow, and all our trees and shrubs with- 

 out exception, there is an outer layer of bark on the stem, 

 and the stem increases in thickness, year by year, by form- 

 ing a new layer just inside the bark and outside the old 

 wood. These stems are therefore called exogenous, that is, 

 outside growers. 



Now, in all Dicotyledonous plants, whether herbs, 

 shrubs, or trees, the stem thickens in this manner, so that 



Dicotyledons are also Exogens. 



CHAPTER XII. 



EXAMINATION OF COMMON PLANTS CONTINUED DOG'S-TOOTH 



VIOLET TRILLI UM IRIS ORCHIS. 



82. Dog's-tOOth Violet. This plant (Fig. 82), which 

 flowers in spring, may be pretty easily recognized by its 

 peculiar blotched leaves. It may be found in rich, moist 

 pasture lands and low copses. The name "Violet" is 

 somewhat unfortunate, because the plant is not in any 

 way related to the true Violets. To obtain a complete 

 specimen requires some trouble, owing to the fact that the 

 root is commonly six inches or so below the surface of the 

 ground ; you must therefore insert a spade or strong trowel 

 sufficiently deep to avoid cutting or breaking the tender 

 stem. Having cleared away the adhering earth, you will 

 find that the roots proceed from what appears to be the 



