DYNAMICS OF THE GROWTH PROCESS 



553 



apical cells 



doscopic growth process than a snapshot can convey any adequate impression 

 of the colorfulness and strategy of a championship football game. Such sec- 

 tions picture us only the results of growth activity, but nothing of the 

 dynamics of the process. 



I. The Cell Division Phase. — Every vegetative stem tip contains one or 

 more cells called initials, the position of which is maintained at or near the 

 apex. All new cells produced at any growing stem or root tip originate 

 directly or indirectly, after intervening cell divisions, from these persistently 

 meristematic cells. In pteridophytes there is generally only one initial, but 

 in angiosperms a number of initial cells 

 are usually present in the growing tip 

 (Fig. ii8). 



One of the two daughter cells origi- 

 nating upon the division of an initial 

 retains its position and meristematic 

 properties. From several to many gen- 

 erations of cell progeny may arise from 

 the other, but eventually all of the off- 

 spring of this daughter cell become 

 mature cells except any which are so 

 located that they become cambium cells. 

 Cell division in apical meristems occurs 

 both longitudinally and in many planes 

 at right angles to the axis, thus giving 

 rise to the typical cylindrical configura- 

 tion of a stem or root tip. Multipli- 

 cation of cells during growth results not only in an increase in the number 

 of cells present but also in some increase in the size of the growing organ, 

 since the combined volume of two daughter cells is usually greater than that 

 of the mother cell. 



The youngest cells of a meristem are called the promeristem. The cells 

 of the promeristem, which includes the initials, are distinguished by certain 

 structural features. Such cells, if observed in the non-dividing condition, 

 are seen to possess thin, delicate walls and are usually approximately isodia- 

 metric. The mass of cells in any meristem appears to be in a highly plastic 

 condition and its continuity is not interrupted even by minute intercellular 

 spaces. There is also some evidence that the intervening walls are actually 

 impregnated with protoplasm. Due to the relatively small size of the cell 

 the nucleus is relatively prominent, although actually it is no larger than in 

 mature cells. The cells of the promeristem are often described as being non- 



FiG. ii8. Longitudinal section 

 through stem tip of grape (Fitis) 

 to show apical cells (initials). Re- 

 drawn from Eames and Mac- 

 Daniels (1925). 



