SPECIALIZED/MODIFIED STEMS

 Stems may be of various forms to serve different functions, such as for food or water storage, for subterranean or aerial anchoring devices, as a means of asexual reproduction, or for climbing. SPECIALIZED/MODIFIED STEMS

 Bulbs – A bulb is a short, underground, food-storage stem axis with extremely reduced internodes and surrounding fleshy scale leaves (a). An onion is the example.  Corm – A solid, bulb-like , underground stem without fleshy scales forms a corm. It has greatly shortened internodes. Examples are the food-storage, reproductive corms of Gladiolus and Crocus. SPECIALIZED/MODIFIED STEMS

 Pseudobulb – Many orchids grow on branches or trunks of other plants (epiphytes). These plants develop a fleshy stem internode with water storage parenchyma. Other orchids, in contrast, are terrestrial (grow in the soil). SPECIALIZED/MODIFIED STEMS

 Rhizome – Found near or below the soil surface, a rhizome is an underground stem that produces scale- like leaves and adventitious roots at the nodes. Rhizomes are found in Iris, Equisetum, and Irish potatoes. SPECIALIZED/MODIFIED STEMS

 Stolon – A lateral stem, “runner”, from the base of a plant develops internodes, and where the apex touches the soil, a new plant with shoots and adventitious roots forms at a node. Strawberry and strawberry begonia form stolons. SPECIALIZED/MODIFIED STEMS

 Succulent stem/ cladophyll – Fleshy water storage stems of parenchyma are found in the spurge and cactus families. In prickly pear cacti, nodes bear leaves that have been reduced to spines. SPECIALIZED/MODIFIED STEMS

 Tuber – Tubers are swollen, underground good storage stems arising at the tips of rhizomes. They bear buds, ”eyes”, at the nodes on the potato tuber. These “eyes” develop into potato sprouts (shoots) when the potato starts to grow. SPECIALIZED/MODIFIED STEMS

 Vine – Vines are stems with long internodes and may have one of various types of climbing devices such as tendrils opposite leaves at a node as in grape, modified stipule tendrils (w) as in green briar, disc-tipped tendrils (t) as in Virginia Creeper, adventitious roots as in philodendron and ivy (q), twining leaf tips as in gloriosa lily, and twining lea petioles as in clematis, or the entire vine may twine as in wood rose.

WOOD AND ITS USES

 In a living tree, 50% of the wood weight comes from water content.  Dry weight is composed of 60-75% cellulose and 15-25% lignin.  Density and Durability are two of the most important characteristics in commercial wood. WOOD AND ITS USES

 Sawing  Radially cut (quartersawed) boards show the annual rings in a side view.  Tangentially cut (plain-sawed) boards show annual rings as irregular bands of light and dark streaks. WOOD AND ITS USES

 Knots  Bases of lost branches covered by new annual rings produced by the cambium of the trunk.  Found in greater concentration in older parts of the log, towards the center. WOOD AND ITS USES

 Wood Products  About half of US and Canadian wood production is used as lumber, primarily for construction.

Veneer - Thin sheet of desirable wood glued to cheaper lumber.  Second most extensive use of wood is pulp.  In developing countries, approximately half of cut timber is used for fuel.

Less than 10% in US and Canada. REVIEW

 External Form of a Woody Twig  Stem Origin and Development  Stem Tissue Patterns  Herbaceous Dicotyledonous Stems  Woody Dicotyledonous Stems  Monocotyledonous Stems  Specialized Stems  Wood and Its Uses Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display