Current Topics in Developmental Biology Volume 52 Series Editor Gerald P. Schatten Departments of Obstetrics–Gynecology and Cell and Developmental Biology Oregon Regional Primate Research Center Oregon Health Sciences University Beaverton, Oregon 97006-3499 Editorial Board ¨ Peter Gruss Max-Planck-Institute of Biophysical Chemistry ¨ Gottingen, Germany Philip Ingham University of Sheffield, United Kingdom Mary Lou King University of Miami, Florida Story C. Landis National Institutes of Health/ National Institute of Neurological Disorders and Stroke Bethesda, Maryland David R. McClay Duke University, Durham, North Carolina Yoshitaka Nagahama National Institute for Basic Biology, Okazaki, Japan Susan Strome Indiana University, Bloomington, Indiana Virginia Walbot Stanford University, Palo Alto, California Founding Editors A. A. Moscona Alberto Monroy Current Topics in Developmental Biology Volume 52 Edited by Gerald P. Schatten Departments of Obstetrics–Gynecology and Cell and Developmental Biology Oregon Regional Primate Research Center Oregon Health Sciences University Beaverton, Oregon San Diego San Francisco New York Boston London Sydney Tokyo Cover photo credit: GUS activity in shoot apex of a two-day-old seedling. From Figure 6 in Chapter 3 “Mechanisms of Plant Embryo Development” by Shunong Bai, Lingjing Chen, Mary Alice Yund, and Zinmay Renee Sung. See text and color plate for further details. This book is printed on acid-free paper. Copyright © 2000 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the Publisher. The appearance of the code at the bottom of the first page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. (222 Rosewood Drive, Danvers, Massachusetts 01923), for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-2000 chapters are as shown on the title pages. If no fee code appears on the title page, the copy fee is the same as for current chapters. 0070-2153/00 $35.00 Explicit permission from Academic Press is not required to reproduce a maximum of two figures or tables from an Academic Press chapter in another scientific or research publication provided that the material has not been credited to another source and that full credit to the Academic Press chapter is given. Academic Press A Harcourt Science and Technology Company 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.academicpress.com Academic Press Harcourt Place, 32 Jamestown Road, London NW1 7BY, UK http://www.academicpress.com International Standard Book Number: 0-12-1