Taxonomy of Flowering Plants - LECTURE NOTES
Hugh D. Wilson

Vegetative Morphology I

The Shoot
 
     Angiosperm structure, and that of vascular plants in general, is the result of meristematic activity.  Meristems are centers of mitotic cell division that produce tissue of the plant body in a manner similar to laying bricks.  The largest (see Pando) and oldest living things are vascular plants and this is due to the nature of the meristem.  Apical or terminal meristems produce root and shoot tissue and these - in essence - are the two 'organ systems' of the vascular plant.  Structures and associated botanical terminology discussed here for both vegetative and reproductive expressions relate, mostly, to the product of the apical or terminal shoot meristem and this pattern of activity defines a fundamental 'framework' of structural organization (see OSU Botany 300, source of the image [left], for more detail).

     As indicated by both size and longevity of some vascular plants, the apical meristem is indeterminate.  Its activity of producing cells, or laying the bricks of the plant body, can continue indefinitely.  The apical shoot meristem also has the capability to produce secondary centers of meristematic activity, lateral meristems.  The lateral meristems provide a structural groundplan for mapping the angiosperm shoot.  The position of lateral or axillary meristems, as either lateral buds or secondary shoots and often subtended (positioned beneath) by a leaf, is known as a node.  Shoot tissue that extends between these points is knowned as the internode.

     Aside from the lateral meristem, another secondary meristematic product of the shoot meristem - the vascular cambium - is significant.  This is a column of dividing cells that produces vascular tissue (xylem and phloem).  Since meristematic activity of the vascular cambium varies according to available resources and conditions during the growing season, its product - wood - often takes the form of 'annual rings'.  Angiosperms with an active vascular cambium are 'woody' and those taking a tree-like form (single stem greater than 4-5 m in height) are arborescent whereas woody angiosperms with a shrub-like aspect (more than 1 shoot, less than 4-5 m) are fruticose.

     Most angiosperms lack a vascular cambium, i.e., they are non-woody herbs or herbaceous.  Both woody and herbaceous angiosperms can show indeterminate apical meristem activity and a perennial life style.  Herbaceous perennials living in temperate or seasonable parts of the world sacrifice 'above ground' tissue during the difficult season but often retain dormant meristems and photosynthate in a tough, underground structure known as a caudex.  However, most herbaceous flowering plants are annuals in that they show an annual cycle of seed germination, vegetative growth, and reproduction with the next generation passing the 'difficult' annual period (Winter, dry season) within the seed or fruit.  Summer annuals germinate in the Spring and pass the Winter as seed whereas Winter annuals germinate in the Fall, flower in the Spring, and pass the dry Summer season as either seed or fruit.  Many key players in the remarkable Texas Spring flora are Winter annuals that are in full vegetative growth during December and January.  Some angiosperms dedicate a full growing season to vegetative growth, sequester photosynthate - often in underground structures - through the difficult season and invest this in a reproductive effort at the start of the second year.  These biennials are often found at the market (carrot, onion, etc.) because we harvest the saved resource prior to its 'intended' use by the plant.  Many herbaceous angiosperm genera of central Texas include species with varying 'life styles' (annual, binnual, perennial) and this can be a factor or 'key character' for identification.  Thus, it is important to insure that the sample collected includes a 'below ground' component.  Also, terms are discrete and biological variation can be continuous and, as a result, difficult to define using specific terms.  Consequently, 'bridging' terms can be applied.  The term suffrutescent, for instance, denotes an herbaceous perennial that shows some woody tendencies, usually at the base.

     The stem 'caul-' refers to the primary product of the terminal meristem, the shoot.  Caulescent denotes a 'normal' shoot with nodes and discernable internodes.  Acaulescent, on the other hand, descibes a situation where the internodes are reduced and, as a result, this shoot is compressed to the point that the plant looks 'stemless', such as head lettuce (Lactuca - Asteraceae) and cabbage (Brassica - Brassicaceae).  Terms relating to various adaptive permutations of the shoot include:

Erect - 'normal' growth toward the sun
Prostrate - a horizontal shoot, flush to the ground
Decumbent - a horizontal shoot, but not flush to the ground - the tips point upward
Caespitose - producing clustered, multiple shoots forming tufts or cushions
Scandent - twining, climbing - vine-like
Scapose - acaulescent but producing an erect, leafless flowering stalk (scape) - Taraxacum officinale

Function modifications of the shoot include:

Rhizome - horizontal, elongate and underground, often thick and fleshy (herbaceous perennial are sometimes rhizomatous see: Cyperus - Cyperaceae)

Tuber - thick and fleshy and usually underground, like a rhizome, but compressed  - via reduction of the internodes - and enlarged as a water/photosynthate storage structure (Potato - Solanum - Solanaceae)

Corm - vertical, compressed and underground, often thick and fleshy (herbaceous perennial)

Bulb - acaulescent with expanded, fleshy leaf bases (Onion - Allium - Liliaceae)
Stolon - horizontal, above-ground, often rooting at the nodes - strawberry (Fragaria - Rosaceae) and St. Augustine grass (Stenotaphrum - Poaceae) are stoloniferous.

Thorn - a hard, sharp-pointed modified shoot (Honey locust -  Gleditsia - Caesalpiniaceae)

Branches of deciduous, woody plants in the Winter show both action of the apical meristem and quite a few useful key characters.  Keys are often available for woody plants in 'Winter condition' for a given flora.  Each Fall the apical meristem is enclosed and protected by a set of terminal bud scales.  These scales fall away each Spring, leaving a ring of bud scale scars on the branch.  Thus, the annual product of a given terminal meristem, nodes and internodes, can be tracked by examining the portion of the branch that lies between the terminal bud and the first set of bud scales scars encountered.  Other features, such as the leaf scar and pattern of vascular bundle scars within the leaf scar, are used for identification in these keys.

Another feature of the shoot that can be used as a key character is the nature of the central tissue or pith.

The pith is normally present as soft, uniform tissue (continuous), but this can be intersected by discs of more dense tissue (diaphragmed) with, in some cases, the 'pithy' soft tissue absent (chambered).
 

The Leaf

     Supported by the stem, leaves are the primary site for photosynthesis in most flowering plants.  Their structure and position on the stem also provides a rich suite of taxonomic characters.  The fossil record suggests that the first angiosperms had simple, alternate, entire leaves.  These terms relate to:  structure, position on the stem (or phyllotaxy), and leaf margin.

STRUCTURE:  (overview)

Simple - blade a single, undivided structure (see compound) - example
Compound - blade divided into substructures (leaflet), each with its own stalk (petiolule)

PHYLLOTAXY (overview)

Alternate - a single leaf at the node
Opposite - two leaves at the node, each usually subtending (immediately below and close to) a lateral bud or shoot
Whorled or verticillate - more than two leaves at the node
Rank - when applied to leaves, denotes rows of leaves along the shoot
Decussate - four ranked or opposite leaves alternating at right angles to those above and below
Cauline  - leaves are those associated with the central shoot
Fascicled - closely clustered or grouped


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