Absorption of water and minerals is enhanced by fragile root hairs, which also form behind the growth zone. These long, threadlike cells greatly increase the roots absorptive surface area. Roots grow and develop from their distal ends. Although the boundaries are not sharp, four developmental zones can be distinguished in a root tip the root cap; the meristematic zone, the elongation zone, and the maturation zone. The root cap protects the apical meristem from mechanical injury as the root pushes its way through the soil. The meristematic zone generates only the primary root, produces no lateral appendages. The elongation zone is the site of rapid and extensive cell elongation. And the maturation zone is the region where cells acquire their differentiated characteristics(Figure354). These four developmental zones occupy only a small part of the root, but growth is still confined to the tip 茎的发育 营养枝的顶端分生组织产生茎,叶和侧芽。顶端分生组织位于茎的最顶端 被未成熟的嫩叶包裹着。和根中的情形相同,茎的生长发育也在顶部发生,但没 有根中那样有序和层次分明,而且茎的生长区域相对根的生长区域要大很多。据 细胞分裂的方向和细胞的大小与活力,茎的顶端分生组织可以分为不同的功能 区。被子植物的茎的顶端分生组织一般具有更明显的分层。通常含有L1,L2和 L3由外至内三层细胞。L1和L2层的细胞进行垂周分裂,也就是新形成的子细胞 间的细胞壁与形成层表面呈垂直方向。L3层的细胞分裂方向就没那么有规律 L1层产生茎的表皮,L2和L3层形成茎的内部组织。茎,叶和侧芽均由这三层细 胞分裂分化形成。 Shoot development The vegetative shoot apical meristem generates the stem, well as leaves and lateral buds(Figure). The shoot apical meristem is located at the extreme tip of the shoot, but it is surrounded and covered by immature leaves. Shoots develop and grow at their tips, as is the case with roots, but the developing regions are not stratified and precisely ordered as they are in the root. Moreover, growth occurs over a much broader region of the shoot than is the case for roots. The shoot apical meristem consists of different functional regions that can be distinguished by the orientation of the cell division planes and by cell size and activity. The angiosperm vegetative shoot apical meristem usually has a highly stratified appearance, typically with three distinct layers of cells. These layers are designated Ll, L2, and L3 where Ll is the outermost layer(FigureP351). Cell the new cell wall separating the daughter cells is oriented at right angles to the F divisions are anticlinal in the ll and l2 layers, that is meristem surface. Cell divisions tend to be less regularly oriented in the L3 layer Each layer has its own stem cells, Ll layer generates the shoot epidermis, and the L2 and L3 layers generateinternal tissue, all three layers contribute to the formation of the stem and lateral organs 叶的发育 叶原基是在茎尖生长点的基部由于增强的细胞分裂形成的突起,叶原基的形成是一个十分有 序的过程,这个因植物种类而异的过程,决定叶子在成熟茎上的排列方式。叶原基的指状突起形 成后,边缘分生组织进行分裂。叶子的片状异面形态通过以垂周分裂为主的细胞分裂逐渐形成 虽然不同植物种类叶片的大小和形态不同,一般来说,叶子的形状,不同于辐射对称的茎,呈扁 平状,有背腹之分。 Leaf developmentAbsorption of water and minerals is enhanced by fragile root hairs, which also form behind the growth zone. These long, threadlike cells greatly increase the root’s absorptive surface area. Roots grow and develop from their distal ends. Although the boundaries are not sharp, four developmental zones can be distinguished in a root tip: the root cap; the meristematic zone, the elongation zone, and the maturation zone. The root cap protects the apical meristem from mechanical injury as the root pushes its way through the soil. The meristematic zone generates only the primary root, produces no lateral appendages. The elongation zone is the site of rapid and extensive cell elongation. And the maturation zone is the region where cells acquire their differentiated characteristics (Figure354). These four developmental zones occupy only a small part of the root, but growth is still confined to the tip. 茎的发育 营养枝的顶端分生组织产生茎，叶和侧芽。顶端分生组织位于茎的最顶端， 被未成熟的嫩叶包裹着。和根中的情形相同，茎的生长发育也在顶部发生，但没 有根中那样有序和层次分明，而且茎的生长区域相对根的生长区域要大很多。据 细胞分裂的方向和细胞的大小与活力,茎的顶端分生组织可以分为不同的功能 区。被子植物的茎的顶端分生组织一般具有更明显的分层。通常含有L1，L2和 L3由外至内三层细胞。L1和L2层的细胞进行垂周分裂，也就是新形成的子细胞 间的细胞壁与形成层表面呈垂直方向。L3层的细胞分裂方向就没那么有规律。 L1层产生茎的表皮，L2和L3层形成茎的内部组织。茎，叶和侧芽均由这三层细 胞分裂分化形成。 Shoot development The vegetative shoot apical meristem generates the stem, as well as leaves and lateral buds (Figure). The shoot apical meristem is located at the extreme tip of the shoot, but it is surrounded and covered by immature leaves. Shoots develop and grow at their tips, as is the case with roots, but the developing regions are not as stratified and precisely ordered as they are in the root. Moreover, growth occurs over a much broader region of the shoot than is the case for roots. The shoot apical meristem consists of different functional regions that can be distinguished by the orientation of the cell division planes and by cell size and activity. The angiosperm vegetative shoot apical meristem usually has a highly stratified appearance, typically with three distinct layers of cells. These layers are designated L1, L2, and L3, where L1 is the outermost layer (FigureP351). Cell divisions are anticlinal in the L1 and L2 layers; that is, the new cell wall separating the daughter cells is oriented at right angles to the meristem surface. Cell divisions tend to be less regularly oriented in the L3 layer. Each layer has its own stem cells, L1 layer generates the shoot epidermis, and the L2 and L3 layers generateinternal tissue; all three layers contribute to the formation of the stem and lateral organs. 叶的发育 叶原基是在茎尖生长点的基部由于增强的细胞分裂形成的突起,叶原基的形成是一个十分有 序的过程，这个因植物种类而异的过程，决定叶子在成熟茎上的排列方式。叶原基的指状突起形 成后，边缘分生组织进行分裂。叶子的片状异面形态通过以垂周分裂为主的细胞分裂逐渐形成。 虽然不同植物种类叶片的大小和形态不同，一般来说，叶子的形状，不同于辐射对称的茎，呈扁 平状，有背腹之分。 Leaf development