THE STRUCTURE OF THE MERISTEMATIC ROOT TIP AND ORIGIN OF THE PRIMARY TISSUES IN THE ROOTS OF VASCULAR PLANTS

Abstract

THERE HAS been considerable activity in recent years in the study of cell lineages and growth in meristems, but in spite of this, comparativelv little attention has been paid to the apical meristem of In Esau's comprehensive review of the literature (1943) on the origin and development of primary vascular tissues in seed plants only four pages out of eighty-one are devoted to procambial and early vascular differentiation in roots. Some seedling anatomy studies have been made (such as Hayward, 1932; Mulendore, 1935; Link, 1941; Hayward and Long, 1942) as well as other developmental studies (Esau, 1940, 1943; Engard, 1944; Allen, 1947), which give attention to the differentiation of tissues in the roots of the plants being considered. It is now generally known that there is a close ontogenetic relationship between the pericycle and vascular tissuesof the root, that the metaxylem initials may be distinguished before the protoxvlem initials even though they mature later in the ontogeny of the root, and that the primary phloem is functional farther down into the root tip than any other vascular tissue (Esau, 1943). In this study, which was begun in 1939, root tips of members of the various groups of vascular plants have been investigated and compared; the purpose being to note similarities and differences in structure and development within and among the various groups, as well as structures -and patterns common to root tips of all the groups. MATERIALS AND METHODs.-The 195 roots studied include. 74 species from 18 families of monocotyledons; 105 species from 48 families of dicotyledons; .11 species from 6 families of pteridophytes, and 5 species from 4 families of gymnosperms. Most of the root tips for this investigation were collected in the field. From such aquatic plants as Saqittaria, Isoetes, Salix, Carex, Juncus, Scirpus, many of the grasses and others, good root tips can be secured at almost any time. From plants of a mesophytic habitat tips can be best secured just following a rain.