DIFFERENTIATION OF PROTOPHLOEM IN THE ANGIOSPERM SHOOT APEX

Abstract

T HE experiments of Curtis(3) lend strong support to the view that the upward translocation of foods, like the downward, is also through phloem. When a shoot is ringed and the part above the ring defoliated, that part makes practically no further growth. Curtis' researches lead one to ask just how far up the shoot the phloem functions. To answer this question one has to ascertain first the condition of this tissue at the developing shoot apex. The vigorous growth of the superficial meristem at the non-green shoot apex giving rise to new leaves and internodes means that the necessary supplies are reaching it from below and at a rate that can maintain such growth. It is true that the superficial meristem cannot receive its supplies immediately from the vascular elements, for between the canopy of embryonic cells and the ends of the vascular tract there exists a gap. This gap is bridged by the forerunner of the vascular tract, the procambial strand. The question whether the elongated cells of the procambium function as a continuation of the vascular elements or not is beyond the scope of the present paper. We are going to present below some observations on the differentiation from the procambium of the earliest tissue to which the function of food conduction is ascribed, i.e. the protophloem.