HIGHER LAND PLANTS 



169 



aerial for leaf support 



internode 



node 



xylem and phloem 

 organized into 

 vascular bundles 



cambium pericycle cortex epidernnis 



cap 



xylem and phloem 

 generally separate 



underground for 

 anchorage and storage 



Figure 11.2 Roots vs. stems. Diagram shows the general arrang 

 ment, but not the details of unlike structures. 



alternate 



opposite 



whorled fascicled 



Figure 1 1 .3 The basic arrangements of leaves upon stems. 



a spiral pattern either up or down the stem. Opposite 

 refers to opposite pairs of leaves at each node, with 

 adjacent nodal pairs usually being at right angles to 

 one another. Whorled designates three or more, usu- 

 ally geometrically inserted, leaves at each node. 



Tracheophyte leaves are of two phylogenetically 

 significant major types, microphylls and megaphylls. 

 Microphylls commonly are small, but not necessarily 

 so. They are diagnosed by a single vein (stele) enter- 

 ing from the stem and forming an unbranched mid- 

 vein that goes to the leaf apex. In contrast to mega- 

 phylls, microphylls usually are smaller and less differ- 

 entiated or modified. 



Megaphyll (or macrophyll) venation ranges from 

 simple forking patterns to a network of small veins 

 that connect the primary veins. The pattern of the 

 primary veins define pinnate, palmate, and parallel 

 venation (Figure 11.4). Pinnate venation consists of 

 a prominent central, longitudinal, main midvein and 



