Leaf morphogenesis in flowering plants.

Leaves, or organs derived from leaves, are the most morphologically diverse structures in a plant (Gifford and Foster, 1989). They occur as prominent photosynthetic structures; as inconspicuous fragments of tissue in tubers, rhizomes, and the stems of many cacti; as nonphotosynthetic storage organs in bulbs; and, in carnivorous plants, as complex structures specialized for catching and digesting small animais. Given this natural diversity, it is not surprising that there is some disagreement about what actually constitutes a leaf (Sattler and Rutishauser, 1992; Rutishauser, 1994). Most leaves have three more or less distinct parts: a leaf base, which may ensheathe the stem; a basal stalk, known as the petiole or rachis; and a dista1 portion, known as the blade or lamina, which is usually green and flat (Figure 1). Two major types of leaves, simple and compound, are distinguished by the shape of the blade. In a simple leaf, the blade is a unitary structure that may have a relatively smooth outline, or it may be moderately or very highly indented. In a compound leaf, the rachis produces a series of leaflets known as pinnae. Some leaves do not have all of these parts, and the form that each part takes in different species or within a single plant is highly variable. Severa1 other features are considered characteristic of leaves. Most leaves have dorsoventral asymmetry. That is, the side of the leaf toward the stem (the dorsal or adaxial surface) and the side oriented awayfrom the stem (the ventral or abaxial surface) are morphologically and anatomically different. Second, leaves are determinate structures, meaning that they do not grow indefinitely. Finally, leaves can usually be distinguished from other leaflike structures by the presence of one or more axillary buds at the junction between the base of the leaf and the stem. It is helpful to divide leaf development into several stages based on the time at which various features of the leaf become determined (Sylvester et al., 1996). During the first stage, the leaf primordium is initiated and acquires its identity as a leaf. During the second stage, the major parts of the leaf become determined and acquire their basic shape, and during the final phase, the histogenesis of the leaf is com-

叶子，或由叶子衍生的器官，是植物中形态最为多样的结构（吉福德和福斯特，1989）。它们作为显著的光合结构出现；作为块茎、根茎以及许多仙人掌茎中不显眼的组织碎片；作为鳞茎中的非光合储存器官；并且在食肉植物中，作为专门用于捕捉和消化小动物的复杂结构。鉴于这种自然的多样性，对于究竟什么构成一片叶子存在一些分歧也就不足为奇了（萨特勒和鲁蒂绍泽，1992；鲁蒂绍泽，1994）。大多数叶子有三个或多或少不同的部分：叶基，它可能包裹着茎；基部的柄，称为叶柄或叶轴；以及远端部分，称为叶片或叶身，通常是绿色且扁平的（图1）。两种主要的叶子类型，单叶和复叶，根据叶片的形状来区分。在单叶中，叶片是一个单一的结构，可能有相对平滑的轮廓，也可能有中度或高度的凹陷。在复叶中，叶轴产生一系列被称为羽片的小叶。有些叶子并不具备所有这些部分，而且每个部分在不同物种中或在同一植株内的形态差异很大。还有几个其他特征被认为是叶子的特性。大多数叶子具有背腹不对称性。也就是说，叶子朝向茎的一面（背面或近轴面）和背离茎的一面（腹面或远轴面）在形态和解剖结构上是不同的。其次，叶子是有限生长结构，这意味着它们不会无限生长。最后，通常可以通过在叶基和茎的连接处存在一个或多个腋芽来将叶子与其他叶状结构区分开来。根据叶子的各种特征确定的时间，将叶子的发育分为几个阶段是有帮助的（西尔维斯特等人，1996）。在第一阶段，叶原基开始形成并获得其作为叶子的特性。在第二阶段，叶子的主要部分确定并获得其基本形状，在最后阶段，叶子的组织发生完成。