88 Growth 



The first phellogen usually arises just under the epidermis, but as axis 

 diameter increases and outer tissues are ruptured, new phellogens appear 

 in the deeper layers. Sometimes these form a single continuous layer of 

 cork which may be peeled off. In other cases the phellogen arises as a 

 localized, often slightly concave sheet which isolates a scale-like patch of 

 tissue. In older stems these phellogens appear in the earlier and more or 

 less crushed and functionless phloem. The phellogen cells obviously are 

 alive and must originate in still living cells of this outer tissue. In many 

 cases, corky cells arise beneath wounds in various regions. In the abscis- 

 sion of leaves and fruits a layer of cork is formed at the region of separa- 

 tion. In both these cases, growth substances (wound hormones or auxin) 

 have been shown to be related to the origin of the cork cambium. Corky 

 layers often show an unusual histological trait in having the new division 

 walls in tangentially neighboring cells laid down directly opposite each 

 other instead of being staggered, thus forming a characteristic stratified 

 structure unlike that of most plant tissues (p. 195). 



From a morphogenetic point of view the most interesting thing about 

 cork-forming cambia is the way in which a continuous layer of such cells 

 may suddenly arise in a mass of old and partially collapsed tissue. A host 

 of dormant cells become embryonic again, link themselves up with neigh- 

 boring cells, and form a cambial layer. In the typical rhytidome form of 

 bark, this may be somewhat irregular in outline and often is not closely 

 parallel to the surface of the organ or to the vascular cambium below. 

 Where cork forms under a wound or just below the epidermis, its position 

 may be explained by its location at a particular point in a physiological 

 gradient, but in these more complex cases such an explanation is less 

 satisfactory. Their origin resembles the way in which a pattern of wall 

 thickenings or a net of fibers (p. 197), which transcends cellular bound- 

 aries, may become differentiated in a mass of tissue. The origin of such 

 phellogen layers is a problem in differentiation which deserves more 

 attention. 



MERISTEMS IN DETERMINATE GROWTH 



Potentially, the plant axis can grow indefinitely in length through the 

 activity of its apical meristems and in width through the activity of the 

 vascular cambium. Actually, of course, growth finally ceases for various 

 reasons, but these axial meristems are essentially indeterminate in their 

 activity. 



The organs of the plant other than stem and root, however— the leaves, 

 floral parts, and fruits— are structures of limited, or determinate, growth. 

 They finally reach maturity and cease to enlarge. In this respect, one of 

 them is much like an animal individual with a definite life cycle of its 



