APICAL MERISTEMS 



21 



the complex phenomena of regeneraiion or regressive differentia- 

 tion, when they are better understood, may be expected to shed 

 important light on the fundamental nature of meristems. (Cf. 

 Bloch, VMl; Sinnott and Bloch, 1941a, 1941b.) 



In the present exercise, a preliminary study will be made of 

 the apical meristems of root and shoot, and of the vascular cam- 

 bium. Further experience with these meristems as well as with 

 the cork cambium will be gained particularly in the exercises 

 devoted to the anatomy of root, stem, and leaf. 



II. Apical Meristems. — 



1. The shoot a per. The classical investigations of C. F. 

 Wolff (1759) on bud development showed that new leaves and 

 new stem tissues are traceable in origin to the delicate tip of the 

 shoot. Wolff designated this region as the "punctum vegeta- 

 tionis," a term which has been rather freely translated as the 

 "growing point." Despite the widespread adoption by anatom- 

 ists of the expression "growing point," this term carries an inac- 

 curate implication and in the present book will be replaced by the 

 more appropriate and non-committal designation of "shoot 

 apex." This decision is based upon the fact that the chief signifi- 

 cance of the so-called growing point is that it represents the 

 region of initiation of the primary organization of the shoot, 

 rather than a localized area or "point" of "grow^th." As a 

 matter of fact, if ' ' growth ' ' is regarded as an increase in size of 

 cells, tissues, and organs, this process is obviously at a minimum 

 in the "growing point." 



Great variation obtains with respect to the form and dimen- 

 sions of the shoot apex of seed plants. As seen in median longi- 

 seetion view% the apex commonly has the form of a mound or low 

 dome. In Elodea, Myriophyllum, and Hippuris, however, the 

 shape of the shoot apex is that of a slender, blunt-tipped cone 

 (cf. Louis, 1935, pp. 126-130 and PI. IX, Figs. 77-78). The apex 

 of dicotyledons with decussate phyllotaxis (e.g., Syringa, Loni- 

 cera, Ligustrum, etc.) is particularly suitable for developmental 

 studies because the initiation of each pair of foliar structures is 

 preceded by a notable and symmetrical expansion of the terminal 

 meristem. Since this process is repeated each time a pair of 

 leaves is produced, the apex exhibits a rhythmical alternation of 



