852 



A TEXTBOOK OF THEORETICAL BOTANY 



flank of the apical dome (Fig. 841). This is a leaf rudiment. Once formed 

 its further development follows a very orderly sequence of cell divisions in 

 new directions. 



The growth of the apex as a whole is therefore rhythmical rather than 

 uniform. It proceeds by a succession of maximal and minimal areas. When 

 the surface has expanded to its maximum area a leaf-fold is formed and the 

 minimal area is regained. Each phase of this rhythm, the period from the 



'^- Stem apex 



E>ud initial 

 Leaf initial 



"* Bud initial 



Fig. 841. — Vibunnim fiagram. Longitudinal section 

 of the apical meristem showing the sequence of 

 leaf initials and origin of the axillary bud initials 

 from the meristem. 



initiation of one leaf primordium to the next is called a plastochron or unit 

 of developmental time, which corresponds spatially to the interval between 

 successive nodes. 



Of course it must be realized that these few mechanical notions provide 

 only the barest bones of an account of development at the apex. There must 

 exist an immense complex of growth controls which determine the details 

 in the development of organs, every one of which has a character peculiar to 

 its species. This has led to the apex being compared to an " organizer," a 

 somewhat mystical conception borrowed from animal embryology, which 

 may only mean that the apex is a nodal point in a framework of chemical 

 and physical gradients in the tissues, which by their interaction determine 



