THE ANGIOSPERMAE : LEAVES 



959 



provides a great number of alternative paths for the flow of Hquids and the 

 mesophyll is much more richly supplied with conducting tissues than in the 

 other group. Moreover, the meshes of the net nearly always enclose at least 

 one branch vein which ends blindly in the middle of the mesh-space, thus 

 ensuring that no part of the mesophyll is more than two or three cells distant 

 from a vascular bundle (Fig. 945). 



The greater physiological efficiency of the dicotyledonous type of venation 

 is clearly seen if a leaf of both types is partly cut across transversely. In the 

 Monocotyledon the severing of the main 

 veins practically cuts off the w-ater supply 

 to the portion of the lamina above the cut, 

 and in dr\^ weather it will w'ither. In the 

 Dicotyledon, however, the danger of such 

 physiological isolation scarcely exists, so 

 long as some of the principal veins remain 

 intact. Notwithstanding the general dif- 

 ference between the two groups in this 

 character of venation, it is not an abso- 

 lute distinction. There are some Dicoty- 

 ledons (e.g., species of Eryngiiim) with 

 parallel venation in narrow leaves, which 

 look typically monocotyledonous (Fig. 

 946). On the other hand the large mono- 



Flc;. 94/. — C'ulocasici aiitiquorum. Mon()Cot>le- 

 donous leaves with a broad lamina and 

 reticulate vemtion. 



Fig. 946. — Eryngium a^avifolium. A di- 

 cotyledonous leaf with parallel venation. 



cotyledonous family of Araceae, 

 among others, has leaves with a 

 broad lamina and network venation 



(Fig- 947)- 



The difference is probably not 



truly based on systematic affinity, 



but rather on the type of the leaf in 



each case. That Monocotvledons 



