Anatomy of Auxin Treated Etiolated Seedlings of Pisum sativum

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Abstract
Etiolated seedlings decapitated, at different ages, in the 3d internode, were treated with auxin paste or with aqueous solns. of auxin of varying concentrations. Collections were made at 8-hour intervals during 164 hours. Swelling and root formation vary in reference to auxin conc. 4 phases of growth occur during the development of a swelling: a) expansion of parenchyma cells and infiltration of auxin paste into the intercellular spaces (0-48 hours); b) formation of a meristem cylinder from inner cortex, starch sheath, pericycle and phloem parenchyma (16-72 hours); c) initiation and elongation of root primordia (56-164 hours); d) lignification of tissues (116-164 hours). In younger seedlings the identity of the endodermis is lost in the meristem cylinder and the root primordia arise from a group of ray cells within this complex. In seedlings treated when older, the endodermal cells do not divide and the primordia are pericyclic or intraperi-cyclic in origin. The starch sheath develops into a "typical" endodermis. with lignified Casparian strips. There is no visible anatomical differentiation in the shoot to account for the approximately whorled vertical distribution of the roots.

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