The Relation of Depth of Planting to the Morphology of the Wheat Seedling

Abstract

The subcrown axis of Triticum vulgare Vill. has been variously called rhizome, subcrown internode and mesocotyl. The term mesocotyl is objectionable for a number of reasons. It has been applied to portions of axis, which in case of oats is hypocotyl and in case of corn and wheat is It is not, therefore, applied consistently to same region of axis. Furthermore, term could be interpreted to mean between cotyledons or possibly a region between hypocotyl and epicotyl. The term should be replaced with correct terms. Normally elongated portion of subcrown axis consists of only one internode but may consist of two or more if seedings are made at depths greater than two or three inches. The fact that more than one internode may occur in subcrown axis would make term rhizome preferable, even though occurrence of more than one internode in elongated portion of axis is rare when seedings are made at usual depths. The term rhizome is commonly defined as an underground stem. From it shoots and adventitious roots may arise. This is true in case of wheat. At seeding depths greater than two or three inches rhizome frequently has distinct nodes from which normal foliage leaves, tillers and adventitious roots arise (Fig. 1). This leaves no doubt that subcrown axis is a rhizome. Normal rhizomes, in addition to elongated internode, have at least one or two other internodes, depending upon interpretation of anatomy of root-stem-transition zone of axis. At greater seeding depths there may be four or five and possibly as many as six internodes in rhizome. The literature of seedling morphology and anatomy of grasses has been reviewed by Van Tieghem (8), Bruns (2), Avery (1) and summarized by McCall (4). Avery (1) reviews briefly historical development of ideas concerning rhizome of cereals. At present time it is accepted that elongated portion of rhizome is an internode, but whether it is second or third internode remains controversial. Avery (1) considers scutellar node to be first node. According to this view elongation of wheat rhizome occurs in second internode. McCall (4) considers scutellum to be the functional cotyledon, divergent from second node. The epiblast is regarded as a vest-gial leaf and to be located at first node or root-stem-transition zone. This view holds that wheat rhizome elongates in third internode, crown developing normally at fourth node. The writer believes that McCall's evidence is convincing. In preliminary studies at Manhattan, Kansas, it has been found that depth of seeding has a marked effect on morphology of wheat seedling. This effect has been noted by Percival (6), Webb and Stephens (9) and