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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.


 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.


 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).


 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.


 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.


 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.


 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.


 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

specialized/modified stems stems may be of various forms ... 3.pdf · specialized/modified stems...

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