BOOK III. 



DEVIATIONS FROM THE ORDINARY NUMBER OF ORGANS. 



To a certain extent the number of the organs of a 

 plant is of even greater consequence for purposes of 

 classification than either their form or their arrange- 

 ment ; for instance, the number of cotyledons in the 

 embryo is made the chief basis of separation between 

 the two great groups of flowering plants, the mono- 

 cotyledons and the dicotyledons. In this one group, 

 moreover, the parts of the flower are arranged in 

 groups or whorls of five ; in the other the arrangement 

 is ternary. In mosses the teeth of the peristome are 

 arranged in fours, or in some multiple of that number. 

 So far as the larger groups are concerned, and also in 

 cases where the actual number of parts is small, the 

 numerical relations above described are very constant ; 

 on the other hand, in the minor subdivisions, and 

 especially where the absolute number of parts is large, 

 considerable variation may occur, so that descriptive 

 botanists frequently make use of the term indefinite, 

 and apply it to cases where the number of parts is 

 large and variable, or, at any rate, not easy to be 

 estimated. 



Considered teratologically, the changes, as regards 

 the number of organs, are readily grouped into those 



