INTRODUCTORY 5 



mines the question of an annual increase in the display of 

 foliage, which means the working power of the body. This 

 character can not be used as a specific test for the two groups ; 

 nor must it be pressed in certain features alone or too rigidly. 

 When intelligently applied, it is probably only second in im- 

 portance to the character supplied by the embryo; but it must 

 be remembered that these prevailing tendencies of the two 

 groups are in some instances exchanged. 



3. The characteristic foliage leaves of Monocotyledons have 

 a closed venation, while in Dicotyledons the venation is open. 

 This character involves many differences in detail. For ex- 

 ample, as a result the Monocotyledon leaf is entire, while the 

 Dicotyledon leaf, with veins ending freely in the margin, is 

 inclined to branch more or less, this tendency expressing itself 

 in the greatest variety of ways from simple teeth to the so- 

 called " compound leaves." * It is also true in general that in 

 Monocotyledons there is a sharp contrast in size between the 

 principal veins of the leaf and the reticulating veinlets; while 

 in Dicotyledons the gradation is so gradual that the reticulation 

 becomes very evident. It may be well to call attention to the 

 fact that while the so-called " parallel " venation may be of 

 service in distinguishing the majority of Monocotyledons in 

 temperate regions, as contrasted with the "pinnate" or "pal- 

 mate " venation of Dicotyledons, it loses its significance when 

 the tropical Monocotyledons are included. The distinctive 

 character of closed or open venation can not be applied to all 

 Monocotyledons and Dicotyledons, and is certainly less gen- 

 eral in its application than the two characters already given. 

 As a character to be used in a cumulative way, however, it 

 deserves prominent mention. 



i. Among Monocotyledons and Dicotyledons with cyclic 

 flowers the establishment of three as the cycle number of the 

 former, and of five or four as the cycle number of the latter 

 is quite distinctive. In fact, the constancy with which these 

 numbers appear is more remarkable than the exceptions. Of 

 necessity, this character is of comparatively limited use, but 

 it is of service among the cyclic families, and also among those 

 families some of whose floral parts are in cycles. The persist- 



* This term should be abandoned for leaves, as has the term "compound 

 flower " for the characteristic head of Compositae. 



