- Meristems are regions of cell division and there are two types of plant meristems. An apical meristem is located at the tip of every shoot and root (Fig. 1). As cells divide in these apical meristems, the shoots and roots elongate as cells are piled one on another. Behind the region of cell division is a region of cell differentiation, where cells enlarge and differentiate into various tissues. In the axil of each leaf is a small apical meristem called an axillary meristem that forms an axillary bud, which usually remains dormant until well after the subtending leaf is fully developed. An axillary bud may remain dormant or develop into a lateral branch or a flower.
Fig. 1. Longitudinal section of shoot tip shows an apical meristem, successively older leaf primordia, and axillary bud primordia.
http://pubs.ext.vt.edu/422/422-025/L_IMG_figure1.jpg
- There are two distinct layers of meristematic tissue within the stem or root responsible for secondary growth, the vascular cambium and the cork cambium (Fig. 2). The vascular cambium is a cylinder of specialized cells, usually five to ten cells thick, running the length of the plant, including the roots, and is responsible for the radial growth of plant parts. Phloem cells are produced to the outside of the cambium and xylem cells are produced to the inside of the cambium. Downward transport of sugars, nutrients, and hormones from the top of the tree to the roots occurs in the phloem tissue. Xylem cells are tube shaped, become hollow and die to form a pipe-like system through which water, hormones and mineral nutrients move from the roots to the top of the tree. Most of the radial growth of woody plants is due to activity of the vascular cambium, but a small amount results from activity in another lateral meristem, the cork cambium, located outside the vascular cambium. The cork cambium (phellogen) together with the cork cells, constitute the periderm: a protective layer of suberized dead cork cells forming the bark. Suberization is the impregnation of cell walls of cork tissue with a fatty substance called suberin. Each season new layers of cells are produced and appear as growth rings when viewed in cross-section. Over time, the xylem cells at the center of the trunk or limb are crushed and become nonfunctional as transport pipes, but they do provide structural support to hold the plant upright. While grafting it is important to line up the cambiums of the scion and the rootstock to ensure a successful graft union.
 Fig. 2. Longitudinal cross-section of a tree trunk shows the vascular and cork cambiums.
http://pubs.ext.vt.edu/422/422-025/L_IMG_figure2.jpg
Buds
- Buds are important to the vegetative and reproductive growth of trees. Fruit tree training and, to a lesser extent, pruning primarily involves bud manipulation. Buds are actually undeveloped shoots. When a vegetative bud is sliced longitudinally during the winter and viewed under magnification, the apical meristem at the tip, leaf primordial (developing leaves), axillary meristems, developing axillary buds, and procambial tissue (tissue that will develop into the cambium) are all visible.
- Buds on fruit trees usually have about seven leaves and initial shoot elongation in the spring results from cell expansion. During late June and July some of the shoot apices will flatten out and develop into flower buds. Flower buds are actually modified shoots and the various flower tissues (petals, stigmas, anthers, etc.) are actually modified leaves. Although the process of switching from vegetative to reproductive buds is not fully understood, hormones that can be influenced by environmental factors, stresses, and plant nutrition control the process.
- There are several things we can do to influence whether or not a bud becomes a flower bud or remains vegetative. In general, factors that favor rapid growth, such as high nitrogen levels in the shoot tissues, inhibit the development of flower buds. Applying growth-promoting plant growth regulators such as gibberellins usually inhibits flower-bud induction, whereas ethylene may promote flower-bud development. Mild stresses such as shoot bending and water stress may also promote flower-bud development.
- Producing annual crops of high-quality fruit requires a balance between reproductive and vegetative growth. Fruit producers use various techniques, including pruning, branch bending, and plant growth-regulator sprays, to manipulate tree growth and flowering. Often these techniques affect bud dormancy, so knowledge of buds and bud dormancy is essential if we are to understand how pruning influences tree growth. It is also important to be able to identify the different types of buds on a tree, especially to distinguish between flower and vegetative buds.
Buds may be classified based on location, contents, or activity.
- Classification by content Several types of buds commonly develop on fruit trees. Vegetative buds only develop into leafy vegetative shoots. Flower buds produce only flowers. Stone fruit trees (peach, nectarine, apricot, plum, and cherry) produce vegetative buds and flower buds. Apple and pear trees produce vegetative and mixed buds. Both leafy shoots and flowers emerge from mixed buds.
i. Classification by location
- Terminal buds are located at the tip of a shoot. On stone fruit trees terminal buds are vegetative buds. Terminal buds on apple and pear trees are usually vegetative; however, some varieties such as Rome Beauty produce mixed buds terminally and are referred to as tip bearers or terminal bearers. Most mixed buds on apple and pear trees are formed terminally on short, less than sixinch, shoots that terminate with a rosette of leaves. These short shoots are called spurs. Lateral buds form in the axils of leaves and are often referred to as lateral buds or axillary buds. On stone-fruit trees, lateral buds may be either vegetative or flower. Nodes on one-year-old shoots may have one to three buds, some of which may be flower buds and others vegetative buds. Flower buds are larger with tips that are relatively round, whereas vegetative buds are small, narrow, and pointed. In the case of apple and pear trees, lateral buds on the previous season's growth are usually vegetative. However, lateral buds on some varieties, especially on the dwarfing rootstocks, may be mixed buds.
ii. Classification by arrangement on the stem
- The bud arrangement influences the arrangement of a fruit tree's branches and thus the tree's shape and how easy it is to manage. A node is the joint on a stem where a leaf is or was attached (Fig. 3). Axillary buds are located in the axis above where a leaf is attached to the stem. In apples there is usually only one leaf per node, whereas three leaves often arise from a node on peach shoots. When a leaf falls in the autumn, a leaf scar remains just below the axillary bud (Fig. 3). Buds are opposite when there are two at the same node but on opposite sides of the stem. Forsythia is an example of a plant with opposite buds. Buds are alternate when there is only one from each node and no one bud is on the same side of the stem as the one next above or below it. Deciduous fruit trees have buds that spiral along a shoot (Fig. 4). The spiraling three-dimensional arrangement of leaves around a stem is known as Phyllotaxy and is expressed as a fraction, where the numerator is the number of turns to get to a leaf directly above another and the denominator is the number of buds passed.
Fig. 3. Section of a limb shows nodes, leaf scars, and different types of buds
http://pubs.ext.vt.edu/422/422-025/L_IMG_figure3.jpg
 Fig. 4. Shoots with alternate arrangement (left) and opposite arrangement (right)http://pubs.ext.vt.edu/422/422-025/L_IMG_figure4.jpg
- In each case the shoot has been headed and the diagram to the right of the arrow indicates how the buds respond to the heading cut.
iii. Classification by activity
- Buds are dormant when they are not visibly growing. When shoots develop around large pruning cuts, they usually are sprouting from dormant buds (Fig. 5). Adventitious buds form irregularly on older portions of a plant and not at the stem tips or in the leaf axils. They form on parts of the root or stem that have no connection to the apical meristems. They may originate from either deep or peripheral tissues. For example, shoots often arise from adventitious buds growing from callus tissue around wounds. Root suckers (vigorous upright shoots developing from the roots) develop from adventitious buds on the roots.
 Fig. 5. Watersprouts developing from adventitious buds around a pruning cut used to lower a tree
http://pubs.ext.vt.edu/422/422-025/L_IMG_figure5.jpg
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