ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN PYRUS MALUS

E vert , R ay F. (U. Wisconsin, Madison.) Ontogeny and structure of the secondary phloem in Pyrus malus. Amer. Jour. Bot. 50(1): 8–37. Illus. 1963.—The secondary phloem of apple consists of sieve‐tube elements, companion cells, phloem parenchyma cells, fiber‐sclereids, and ray parenchyma cells. The sieve‐tube elements are generally long, slender cells with very oblique end walls and much‐compounded sieve plates. All sieve‐tube elements initially possess nacreous thickenings. Similar wall thickenings were observed in the differentiating fiber‐sclereids and xylem elements. Of the 245 sieve‐tube elements critically examined, 242 were associated with companion cells. All of the companion cells were shorter than their associated sieve‐tube elements. Young companion cells possess slime bodies which later become dispersed. Callose is often found on the sieve‐tube element side of the common wall between sieve‐tube element and companion cell. In several collections, callose was found on both sides of that wall. The parenchyma cells are of 3 types: crystal‐containing cells; tannin‐and/or starch‐containing cells; and those with little or no tannins or starch. Any type parenchyma cell may be on to genetically related to a sieve‐tube element, that is, may be derived from the same phloem initial as the sieve‐tube element. Morphologically, the phloem parenchyma cells intergrade with the companion cells, the tannin‐ and starch‐free parenchyma cells often being difficult to distinguish from companion cells. Most of the tannin‐ and starch‐free parenchyma cells collapse when the contiguous sieve‐tube elements become nonfunctional. The fiber‐sclereids arise from parenchyma cells which overwinter on the margin of the cambial zone and differentiate in nonfunctional phloem.