METABOLIC GRADIENTS 77 



In the development of the plant the growing tip is 

 the first part of the individual to become distinguishable, 

 and from it other parts arise. In the moss embryo in 

 Fig. 18 the growing tip is already present as the upper- 

 most cell and other cells have arisen from it in an orderly 

 way. In the higher plants the growing tip is not usually 

 localized until later stages. In Gingko, for example, the 

 growing tip of the plant is not yet distinguishable at 

 the stage of Fig. 19, although the small-celled region is 

 the growing tip of the whole proembryo and in this the 

 growing tip of the plant-stem later appears. In certain 

 algae the major axial gradient in the egg is apparently 

 determined by external factors, such as light, but in 

 most plants this gradient is determined by the relation of 

 the egg to the parent body, the growing tip of the plant- 

 stem arising from the apical region of this gradient. 



The vegetative stages of certain liverworts and the 

 sexual generation of various ferns show a high degree of 

 bilateral symmetry and often consist, at least during the 

 earlier stages of their growth, of single elongated flattened 

 individuals (Figs. 23, 24) with a growing tip, a, at one 

 end, often with a thickened longitudinal midrib and with 

 root-like outgrowths on the ventral surface, the surface 

 facing the substratum as the plant grows. In many 

 cases these individuals undergo division by branching or 

 by the formation of buds on the surface in later stages. 

 In these plants, as in bilaterally symmetrical animals, 

 three axes — longitudinal, transverse, and dorso-ventral — 

 are distinguishable; in other words, order is apparent in 

 three directions. Various indications of gradients in 

 activity appear in the same directions. As regards the 

 major axis, the rate of cell division and growth is highest 



