198 STRUCTURAL BOTANY 



and their course, in the stem, at any rate, less simple. 

 In the leaf we have, as a rule, what is called " parallel 

 venation," which means that there are a number of 

 large longitudinal bundles with more delicate trans- 

 verse connections. The presence of numerous main 

 bundles in the leaf goes together with the complex 

 conducting system of the stem. The fact that the 

 leaf generally has a broad base also helps to explain 

 the number of the vascular bundles which enter it. 



(3) The closed structure of the vascular bundles, 

 and the general, though not universal, absence of 

 secondary thickening in Monocotyledons, have been 

 sufficiently dwelt upon above. These characters render 

 necessary the greater complexity of the primary tissues, 

 and also explain the absence of a persistent tap-root, 

 which without cambium cannot satisfy the needs of 

 the plant as it grows in bulk. 



(4) Perhaps the most constant character of all is 

 found in the structure of the embryo, with its single 

 terminal cotyledon and lateral growing-point. It is 

 this peculiarity, more than any other single character, 

 which separates the Monocotyledons from the Dicoty- 

 ledons as a distinct Class. We have now considered 

 the distinctions between the two Classes ; their points 

 of agreement scarcely need to be emphasised. 



The general morphology is alike in both : stem, 

 leaf, and root show the same distinctive characters, 

 both internal and external. 



The structure of the tissues, as distinguished from 

 their arrangements, is similar in the two Classes. 



The most striking agreement, however, is in the 

 floral organs. Speaking broadly, we may say that 



