66 



Growth 



plant seem to be determined. The period between the initiation of two 

 primordia (or two pairs, if the leaves are opposite) is termed a plasto- 

 chron (Askenasy, 1880; Schmidt, 1924). These periodic changes in the 

 meristem can be seen best in opposite-leaved forms. As the two primordia 

 begin to appear, the apical dome between them becomes relatively flat. 

 When they have developed further but before another pair appears, the 

 dome bulges upward again and reaches its maximal surface area. In the 

 lower vascular plants the primordium arises from one or more of the 

 surface cells of the meristem but in the higher ones it develops as a 

 swelling on the side of the apex at the base of the dome, generally as 

 the result of periclinal divisions in one or more layers below the surface 

 one. The term plastochron index, for the interval between corresponding 

 stages of successive leaves, has been proposed (Erickson and Michelini, 



Shoot Tip during Rutmg Prion 



Shoot Tip during Second Growth Phatt 



Fig. 4-12. Diagram of shoot tip of Abies concolor. At left, during resting phase. At 

 right, during second growth phase. That portion of the shoot apex above plane abed, 

 which marks the level of the youngest leaf primordium, has a very different zonal 

 topography in the two stages. 1, zone of apical initials; 2, mother-cell zone; 3, 

 peripheral zone; 4, zone of central tissue. ( From Parke. ) 



1957 ) as a better measure of the stage of development of a growing shoot 

 than is its chronological age. In many plants the shape and structure of 

 the meristem change somewhat with the season (Parke, 1959; Fig. 4-12). 



The phyllotaxy of a shoot is determined by the arrangement of the 

 leaf primordia around the axis. This phyllotactic pattern has been studied 

 developmentally in the meristem, both through observation and experi- 

 ment, by a number of workers (Chap. 7). The regularity and precision 

 with which the leaf primordia arise at the shoot apex are evidence that 

 this region has a high degree of organization. 



Branches are formed from meristems arising in the axils of the leaf 

 primordia. They are at first much smaller than the main apical meristem 

 but do not differ essentially from it. Whether the potentially meristematic 

 tissue here will grow into buds and whether these buds will produce 

 branches are dependent in most cases on the stimulatory or inhibitory 



