Stems

The stem is an aerial axis of the plant that bears leaves and flowers and conducts water and minerals from the roots and food from the site of synthesis to areas where it is to be used. The main stem of a plant is continuous with the root system through a transition region called the hypocotyl. In the developing embryo, the hypocotyl is the embryonic axis that bears the seedling leaves (cotyledons).

In a maturing stem, the area where a leaf attaches to the stem is called a node, and the region between successive nodes is called an internode. Stems bear leafy shoots (branches) at the nodes, which arise from buds (dormant shoots). Lateral branches develop either from axillary, or lateral, buds found in the angle between the leaf and the stem or from terminal buds at the end of the shoot. In temperate-climate plants these buds have extended periods of dormancy, whereas in tropical plants the period of dormancy is either very short or nonexistent.

The precise positional relationship of stem, leaf, and axillary bud is important to understanding the diversity of the shoot system in angiosperms. Understanding this relationship makes it possible to identify organs such as leaves that are so highly modified they no longer look like leaves, or stems that are so modified that they resemble leaves.

Branching in angiosperms may be dichotomous or axillary. In dichotomous branching, the branches form as a result of an equal division of a terminal bud (i.e., a bud formed at the apex of a stem) into two equal branches that are not derived from axillary buds, although axillary buds are present elsewhere on the plant body. The few examples of dichotomous branching among angiosperms are found only in some cacti, palms (Arecaceae), and bird-of-paradise plants (Strelitziaceae).

The two modes of axillary branching in angiosperms are monopodial and sympodial. Monopodial branching occurs when the terminal bud continues to grow as a central leader shoot and the lateral branches remain subordinate—e.g., beech trees (Fagus; Fagaceae). Sympodial branching occurs when the terminal bud ceases to grow (usually because a terminal flower has formed) and an axillary bud or buds become new leader shoots, called renewal shoots—e.g., the Joshua tree (Yucca brevifolia; Asparagaceae). Plants with monopodial growth are usually pyramidal in overall shape, while those with sympodial growth often resemble a candelabra.

By combining monopodial and sympodial branching in one plant, many different tree architectures have evolved. A simple example is found in dogwoods (Cornus; Cornaceae), where the main axis is monopodial and the lateral branches are sympodial.

Very different plant forms result from simply changing the lengths of the internodes. Extreme shortening of the internodes results in rosette plants, such as lettuce (Lactuca sativa; Asteraceae), in which the leaves develop but the internodes between them do not elongate until the plant “bolts” when flowering. Extreme lengthening of the internodes often results in twining vines, as in the yam (Dioscorea esculenta; Dioscoreaceae).