170 



COMPENDIUM OF GENERAL BOTANY. 



pyramidal cell. In the latter case each segment may develop a 

 leaf (see Fig. 100, diagraniatic apical views). If each successive 

 segment develops more strongly on one side, there will be produced 



Fig. 99. 



Fig. 100 B. 



a spiral arrangement of the leaves (see Fig. 100 B, diagramatic 

 apical view). In still other cases the halves of each segment (either 

 side by side or above each other) may develop leaves or branches. 

 (Leftgeb has studied the development of the mosses more par- 

 ticularly.) 



In regard to the position, inclusive of the succession of develop- 

 ment, of lateral organs we may distinguish the following cases : 



I. Organs are in'egnlar as to origin and position. Example: 

 trichomes upon other organs. 



II. The origin and position of organs may be in longitudinal 

 rows, as is the case in secondary roots formed from the perieam- 

 bium. The number of rows is dependent upon the number of 

 primordial xylem-groups. The succession of their development is, 

 as a rule, acropetally, therefore the youngest branch is always nearest 

 the apex of the root. Longitudinal rows of lateral organs, such as 

 secondary roots and leaves, also occur in creeping (" dorsiventral ") 

 organs. 



III. Frequently organs originate and are arranged in whorls, as 

 is seen in the case of leaves and branches. Two or more lateral 



