62 Growth 



useful means of determining their particular meristematic origin. Evidence 

 from these sources shows that a particular tissue may come from one 

 meristematic layer in one plant and from another in another plant. 



Hanstein's histogen theory has largely been superseded by another, 

 first proposed by Schmidt (1924; Fig. 4-7). This recognizes an outer zone 

 of layered cells, usually from one to four cells thick, the tunica, covering 

 a core of unlayered cells, the corpus. The tunica-corpus theory does not 

 maintain that either of these regions produces specific organs or tissues 

 but describes the common type of organization of the shoot apex (see 

 Reeve, 1948). 



The significance of layering in the meristem has often been overem- 

 phasized. Whether or not a layer is formed depends on the plane of di- 

 vision of the meristematic cells. When the apical initials always divide 



Fig. 4-8. Longitudinal section of shoot apex of Torreya californica, showing almost 

 complete absence of layering. ( From Johnson. ) 



anticlinally they obviously will produce a layer, and its growth will be 

 entirely growth in surface. Where divisions occur in other planes or ir- 

 regularly, layers are not produced. Specific factors such as mechanical 

 pressure (p. 49) which influence plane of division may thus determine the 

 presence and number of layers. If the central region of the meristem 

 is growing faster than the surface, the latter will be subjected to pres- 

 sure, its cells will tend to divide parallel to the direction of that pressure, 

 and a layer will be formed. Perhaps this is the only real significance of 

 the layered structure. It is noteworthy that the shoot meristems of many 

 gymnosperms (Korody, 1938, and others; Fig. 4-8) show little or no 

 layering but that they produce structures in a perfectly normal fashion. 

 Layering as such, in the sense of marking out particular regions of the 

 meristem that are significant morphogenetically, seems to be of much 

 less importance than many workers have regarded it. 



