Plant Biology - The Shoot System Stems - Quick Guides | Biology

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Study GuidePlant Biology–The Shoot System: Stems1. Characteristics of Shoot Systems1.1What is the shoot system?Theshoot systemis the easily visible, above-ground part of a flowering plant. It includes thestem,leaves,flowers, andbuds. Together, these parts help the plant grow upward, make food, andreproduce.1.2Structure of the stemLeaves are attached to the stem at specific points callednodes.The part of the stembetween two nodesis known as aninternode.Theleaf axilis the upper angle formed between the leaf and the stem at a node.Inside each leaf axil, there is usually anaxillary (lateral) bud. These buds can grow into:•Vegetative branches(leafy shoots), or•Flowers(reproductive structures).1.3Terminal bud and origin of the shootAt the tip of the main stem and its branches is theterminal bud. This bud containsapical meristemtissue, which is responsible for the increase in length of the shoot.The shoot begins to form during theembryonic stageof the plant. It develops at the endoppositethe root. As it grows, the shoot forms ashoot apex, which is structurally different from the root apex.

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Study Guide1.4Shoot apex and bud formationTheapical meristem, which is the growing point of the shoot, is surrounded by young, developingleaves calledleaf primordia.In the axils of these leaf primordia arebud primordia.There are two types of buds:•Vegetative buds–grow into leafy branches•Reproductive buds–contain early flower tissues and develop into flowers1.5Role of auxins and apical dominanceAuxinsare plant hormones produced mainly at the shoot tip. These hormones usuallysuppress thegrowth of lateral (axillary) buds, keeping them dormant for some time.•Whenapical dominance is strong, the terminal bud grows faster than lateral buds. Theplant becomescone-shaped, with one main stem and shorter side branches.•Whenapical dominance is weak, axillary buds grow soon after the main shoot elongates.This results in abushy plantwith many branches and no single dominant stem.Auxins produced in theleaf primordiaalso control theelongation and differentiation of primarymeristems, helping the shoot grow in an organized manner.

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Study Guide2. Primary Growth of Stems2.1. How Stems Grow in Length

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Study GuideJust like roots,stems grow longer from their tips. This growth happens because cells at the tipdivide and then elongate.In stems (but not roots), the youngest cells at the tip are arranged intotwo zones:Tunica•Theouter layerof the stem tip•Cells divideperpendicularto the stem axis•Formsseveral layersthat cover the surface•Helps in forming theouter tissuesCorpus•Theinner regionof the stem tip•Cells dividein all directions•Produces thebulk (inner mass)of the stemTogether, the tunica and corpus make up theapical meristem, which is responsible forprimarygrowth (increase in length).2.2Formation of Primary Meristems and TissuesCells produced by the tunica and corpus continue dividing and becomethree primary (transitional)meristems:Primary Meristems → Primary Tissues

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