THE PLANT BODY 



31 



and was replaced by the tunica-corpus theory (Fig. 10). This new 

 theory stimulated research in shoot apices of many taxa during the 

 following decades. 



According to the tunica-corpus theory, the shoot apex consists of 

 meristematic tissues arranged more or less distinctly in two major parts 



7-K/»^ r 



TrJ=>^ ^-^ 



•ry^£^ TJX 



T-yper iv 



Type v 



Txpfr iri 



T-ype v/r 



Fi 



of 



ig. 10. Diagrams of longitudinal sections of shoot apices showing types of 

 organization in vascular plants. S, surface meristem; M, mantle; MO, central mother 

 cells; C, cambium-like zone; SA, subapical initials; CM, central meristem; P, 

 peripheral meristem. {After Popham.) 



— a central core, or corpus, sheathed by one or more external layers, 

 the tunica (Fig. 11). In descriptions of the earlier years under this 

 theory, interpretations were made rather rigidly. But it soon became 

 evident that limitation of zones is not always clear and was sometimes 

 "found" in different places by different investigators; that the number 

 of layers in the tunica, considered characteristic of a species, varies with 



