PLANT CELL GROWTH AND NUTRITION 



483 



the dark. Under these circumstances the youngest leaves and the shoot 

 apex grow, and they receive salts and bromide both from the roots and 

 from the lower leaves with which they may be in direct contact. Thus, 

 as leaf No. 6 ( Figure 13 ) came into its active growth period, it tended 

 to deplete leaf No. 1 of solutes, which in turn could be replaced from 

 the roots when the conditions permitted this. In the 2/5 phyllotaxis of 

 Cucurbita pepo (Figure 14), leaf No. 6 connects very directly via a 

 main cauline bundle with leaf No. 1 below. 



Thus one sees the internal nutritional economy of the plant body 

 to be responsive to the interplay of the factors in the growing point of 

 the shoot that regulate and coordinate its development. The facts of 

 anatomy and developmental morphology are here neither redundant 

 nor old-fashioned. Indeed, the problems of nutrition become meaning- 

 ful only as they are interpreted through the metabolism that furnishes 

 the energy and drive for the endergonic processes of growth and also 

 through the morphogenetic responses that are the unfolding expression 

 of the stimuli to growth and organogenesis in the shoot apex. If plants 

 are actively accumulating salts or solutes, it is a sound axiom to look 

 for the places where they are most actively growing, and when move- 

 ment and redistribution occur, it is also sound to see the driving force 

 that regulates the pace of movement through the use of metabolic 

 energy in the growing, accumulating cells at the "sink." For this reason 

 the leaf in the light, quite apart from its "transpiration pull," exerts its 



Vascular system 



= Cauline bundles 



r<^ Common bundles 



C6 



Leaf traces 

 Nodal anatomy 



Leaves 

 1 and 6 



^ 



rOi> 



/=>1 



I II IIIIVV V 



Figure 14. The vascular pattern of Cucurbita pepo, showing direct con- 

 nection of leaves 1 and 6 via cauline bundles. Each leaf receives three leaf 

 traces w hich fuse into a network at the nodal plate. Of these three traces, one 

 springs directly from a cauline bundle. (From Steward, 1954.) 



