70 



Growth 



all cases studied in the ferns), vascular bundles were regenerated in the 

 former pith tissue and became connected basipetally with the vascular 

 system of the axis below. A normal shoot was thus restored. Similar 

 results were obtained by Wardlaw (1950) with Primula. Furthermore, 

 Ball ( 1946 ) succeeded in growing complete plants from isolated apices 

 in sterile culture, and this has now been done repeatedly in other cases, 

 both with ferns and seed plants ( see Wetmore, 1954 ) . 



As a result of such experiments it is now generally agreed that the 

 shoot apex is totipotent and independent of the rest of the plant. This is 

 hardly surprising since many— perhaps all— cells are totipotent under 

 favorable conditions. The subjacent tissue must have some influence on 

 the apex, however, since from below there come into it not only water 



Fig. 4-14. Longitudinal median section through the stem apex of Dryopteris, show- 

 ing how the region around the apical cell ( a ) has been isolated by deep cuts extend- 

 ing through the vascular tissue (v.t. ). (From Wardlaw.) 



and nutrient materials but specific substances of morphogenetic im- 

 portance. The induction of flower buds at the meristem, for example, 

 results from a hormone brought thither from the leaves. There evidently 

 must be a reciprocal relation between the shoot meristem and the axis 

 below it for both are parts of the same integrated organic system. 



The shoot apex has a morphogenetic role which goes beyond regenera- 

 tion, however, for it exerts a strong influence on the differentiation of 

 tissues and organs in the region adjacent to it. By a long series of ex- 

 periments reported in many papers (see Wardlaw, 1952a, 1952/7, and 

 Cutter, 1958) Wardlaw and his colleagues have made important con- 

 tributions to a knowledge of this differentiation. Much of the work was 

 done on the apical meristems of ferns. In Dryopteris, Wardlaw (1949&) 

 determined the region where the next leaf primordium would arise 



