Phloem Studies in the Pteridophytes, Part I. Equisetum

Abstract

Xylary tissues of the pteridophytes have beeii studied in detail by several workers, but phloem tissue has received very little attention. The probable reason for this is the difficulties encountered in studying the anatomical structure of the phloem, for example in cutting perfect radial longitudinal and cross sections of sieve elements showing the details of sieve areas and in staining the tissues. Little is known about the phloem anatomy of Equisetum. I have studied phloem in the rhizomes and aerial shoots of E. hyemale, E. telmateia, E. arvense, and E. giganteum. This paper concerns only the latter two species. The purpose of this investigation was to gather data on Equisetum phloem that could be compared with presumably comparable tissue described in some American calamites (Agashe, 1964). Johnson (1933), in discussing the origin and development of certain tissues in E. scirpoides, mentioned the occurrence of sieve plates with small pores on the lateral walls of the sieve cells. Golub and Wetmore (1948) described the development of phloem in E. arvense. According to them, protophloem sieve cells may be 3 mm long or more. Their longitudinal walls have many, transversely elliptical, faintly staining sieve areas; such areas are also found on the oblique end walls, but they do not seem to form distinct sieve plates. Mletaphloem sieve cells are longer, larger in diameter, and have densely granular cytoplasm and very thick walls; sieve areas are more common on the end wall in metaphloem than in protophloem. There appears to be some confusion in the literature regarding the definitions of sieve cells and sieve tubes. The presently accepted definitions, according to Esau, Cheadle, and Gifford (1953), who quote Cheadle and Whitford, state that a sieve tube consists of sieve elements joined end-to-end to form a vertical tube-like structure in which sieve areas are more highly specialized on the 74