Exercise 111 



MERISTEMS 



I. Introduction. — A meristem may be defined as a specific re- 

 gion in the plant body where cells are engaged chiefly in division 

 and enlargement. Meristems thus represent emhrifonic areas and 

 can be conveniently classified according to position as apical 

 meristems and lateral meristems. The former type is illustrated 

 by the apex or "growing point" of the root and the shoot, the 

 latter by the vascular and cork eambia. In addition, many 

 authors recognize "intercalary meristems" which are said to 

 occur hetween areas of permanent or mature tissue, as for exam- 

 ple at the base of the leaf in certain monocotjdedons. A more 

 critical study of the so-called "intercalary meristems" is urgently 

 needed, however, and attention will be again directed to this 

 problem in the exercise dealing with the leaf (Exercise XIII). 



The maintenance of meristems at certain restricted regions 

 of root and shoot is responsible for the distinctive "open system" 

 of growth characteristic of all higher plants. This consists in the 

 continuous formation of new tissues and new organs throughout 

 the life of the individual. The magnitude of this type of "un- 

 limited" ontogeny is most impressively shown in long-lived 

 woody perennials in which each season's growth is accomplished 

 by the formation of uew shoots, reproductive structures, and 

 roots, as well as by an increase in diameter of the older roots and 

 stems. When the open system of growth is further examined, 

 it becomes clear that there are varying degrees of "permanence" 

 in the various meristems of a plant. Thus the apical meristems 

 of the shoot and root in many ]ierennials appear capable of indefi- 

 nite life and activity. Actually, of course, various factors such 

 as malnutrition, insufficient water, injury, etc., may result in the 

 death of the shoot or root apex. Furtliermore, the phenomenon 

 of correlation, in this case involving the relative growth of main 

 stem or main root as compared with its laterals, becomes a com- 



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