46 ELEMENTS OF STRUCTURAL BOTANY. 



'^' constructed on the same plan. The em- 



bryo of the bean is dicotyledonous also. 

 But you will observe that in these cases 

 the embryo occupies the whole of the inte- 

 rior of the seed. In describing the seed 

 of the Buttercup, it was pointed out that 

 the embryo occupies but a very small 

 Pig. 71. space in the seed, the bulk of the lat- 

 ter consisting of albumen. Seeds like those of the But- 

 tercup are therefore called alhuminons seeds, while thoseof 

 the Bean and Pea are e.valhiuninons. But, notwithstand- 

 ing this difference in the structure of the seed, the emUryo of 

 the Buttercup, when examined under a strong magnifier, 

 is found to be dicotyledonous like the others. In shoit, 

 the dicotyledonous embryo is a character common to all 

 the plants we have examined — common, as a rule, to 

 all i^lants possessing the other characters enumerated 

 above. From the general constancy of all these char- 

 acters, plants possessing them are grouped together in 

 a vast Class, called Dicotyledonous plants, or, shortly, 

 Dicotyledons. 



60. Besides the characters just mentioned, there is 

 still another one of great importance, which Dicotyle- 

 dons possesses in common. It is the manner of yroivth 

 of the stem. In the Willow, and all our trees and shrubs 

 without exception, there is an outer layer of bark on the 

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

 by forming a new layer just inside the bark and outride 

 the old wood. These stems are therefore caDed exofjenom, 

 that is, outside groivers. 



Now, in aU dicotyledonous plants, whether herbs, 

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

 that Dicotyledons are also Exogens. 



