Gross Phloem Anatomy in the Roots of selected C4, C3, and C4‐C3 Intermediate Poaceae Species1

Is the anatomy and physiology associated with the C4 carbon pathway exclusively a feature of the leaf (photosynthate source)? To assist in establishing a more inclusive knowledge of sourcesink relations, qualitative and quantitative aspects of mature root phloem anatomy were compared between related C4, C3, and C4‐C3 intermediate species, (Triticum aestivum L., Zea mays L., and five Panicum spp.). In all species each phloem bundle was contiguous with the pericycle and separated from any tracheary element by one or two rows of stelar parenchyma. The number of cells between phloem bundles was essentially constant and the spatial relationships among the bundles and the cortex and surrounding stelar tissues were similar in the species studied. One protophloem sieve‐tube element, two companion cells, and one metaphloem sieve‐tube element constituted the basic cellular composition of a bundle. The exception was Z. mays L., in which two to six metaphloem sieve‐tube elements and one or two phloem parenchyma cells were usually present. There were significant differences among these species, but these differences were best explained in terms of species specificity rather than by photosynthetic characteristics.