Anatomy of Auxin Treated Etiolated Seedlings of Pisum sativum

Abstract

1. Three series of etiolated seedlings of Pisum sativum were grown, decapitated in the third internode, and treated with auxin paste or with aqueous solutions of auxin. The treated seedlings were collected at eight hour intervals and observed for 164 hours. Controls were grown under exactly similar conditions. 2. The general results of auxin treatment, swelling and root formation, vary in extent in relation to seedling age and auxin concentration. 3. The form of the swelling varies in reference to auxin concentration. 4. Four phases of growth are recognized during the formation of a swelling, each characterized by a dominant activity: (a) expansion of parenchyma cells and infiltration of auxin paste into intercellular spaces (0-48 hours); (b) formation of a meristem cylinder from division of inner cortex, starch sheath, pericycle, and phloem parenchyma tissue (16-72 hours); (c) initiation and elongation of root primordia (56-164 hours); (d) lignification of tissues (116-164 hours). 5. The root primordia vary in origin in younger and older stems. In younger seedlings the identity of the endodermis is lost in the meristem cylinder complex. The root primordia are traceable to a group of ray cells within this complex. In older seedlings the endodermal cells do not divide and the primordia are therefore pericyclic and intra-pericyclic in origin. The endodermis is traceable as a primordial sheath. Casparian strips may or may not be lignified. 6. The roots arise approximately in whorls at successive levels on the stem. There is no visible anatomical differentiation to account for this distribution.