INTRODUCTION 19 



plicatiiifr factor. Another example of a theoretically ' ' indefinite 

 or "})ei-nianent" nieristem is furnished by the vascular camhium, 

 which may continue to produce annual increments of phloem and 

 xylem for hundreds or, in the ^'enus Sequoia, thousands, of years. 

 Obviously, the maintenance of an indeterminate type of meristem, 

 such as a shoot apex or the cambium, requires that there shall be 

 a continuous new formation or " reii'eneration " of the meristem 

 as development takes place. In other words, a certain restricted 

 portion of such meristems remains indefinitely in the embryonic 

 state and does not pass into the zone of maturation. In contrast, 

 the meristems of determinate organs, such as leaves and fruits, 

 function for only a comparatively restricted period, and even- 

 tually all embryonic tissue passes into a state of maturity. Evi- 

 dently these differences in the functional life of meristems are 

 of fundamental morphological importance, but the nature of the 

 factors, genetical and physiological, which control them are very 

 poorly understood at present. 



From an histological standpoint, a meristem is a "tissue" 

 composed of "undifferentiated" or meristematic cells. Accord- 

 ing to the classical viewpoint, which is still retained in many 

 textbooks, the tissue composing meristems possesses certain dis- 

 tinctive juvenile characteristics which seem to demarcate it from 

 the various types of functionally-mature "permanent" tissues. 

 Among the "negative" characters usually assigned to meri- 

 stematic tissue are: (1) the absence of intercellular spaces; (2) 

 the absence of thickened or pitted walls; and (3) the absence of 

 prominent ergastic materials in the cytoplasm. If, however, 

 undue emphasis is placed upon such morphological features, a 

 narrow and rather arbitrary concept of "meristematic tissue" 

 inevitably results. A good example of the restricted concept of 

 meristem is found in Priestley and Scott's recent (1938, p. 208) 

 "An Introduction to Botany." They state: "It is customary to 

 speak indifferently of any dividing tissues of the shoot apex as 

 meristematic, but in view of the fundamental character of the dif- 

 ferences in cell behavior, it is proposed in this book to restrict the 

 terms meristem and meristematic to the dense cells which are 

 devoid of obvious water vacuoles and have no intercellular spaces 

 between them, whilst the vacuolating cells will be spoken of as 



