PLANT ORGANS: Root, stem and leaf.
The general structure of
the stem is essentially
a mirror of the root, with apical meristem at the tip, the three
primary meristems just behind the apical meristem and the three
ground tissues and more mature structures down lower, behind the
three primary meristems.
The other plant organ, the leaf, branches off from the stem--not from the root.
The axillary buds are located in the leaf axils.
* leaf blade
* leaf midrib
* leaf axil (with axillary bud)
* node
* internode
* terminal bud (with apical meristem)
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And now, a look at a WOODY STEM
1. True, botanical wood is produced by a vascular cambium, and consists of XYLEM, arranged in concentric rings.
2. The rings of a woody stem are visible because of differences in the lumen size between SPRING WOOD and SUMMER WOOD. Spring wood is formed early in the year, and has large lumens. Summer wood is formed later, and has small lumens.
In autumn/winter, no significant growth takes place. Thus, the initiation of springwood growth in the growing season creates a very visible delineation between the years' growth.
3. Some apparently woody plants (such as palm trees) do not produce true wood (xylem rings), as they lack a vascular cambium. We'll learn more about them later!
4. Springwood and summerwood should NOT BE CONFUSED WITH Heartwood and Sapwood!
Springwood & summerwood are both found in a single year's growth ring.
Heartwood & sapwood can only be seen in the cross section of an ENTIRE WOODY STEM.
Heartwood: central xylem that is now clogged with resins, dark and no longer conducts water.
Sapwood: outer core of xylem that conducts water and is not clogged. It is generally lighter in color than heartwood.
More about Stems...
(Note: adventitious roots are roots which sprout from aboveground stems before penetrating the soil. They should not be confused with stems! e.g. - Ficus aerial roots that you saw out in the Arboretum.)
Know from figure 6.14 of your text...
1. rhizome - underground stem. Typical of ferns and some other plants
2. tuber - underground storage stem. (e.g. potato)
3. tendril - typical of climbing vines; these respond to touch and grow around supporting items
4. runner - aboveground propagative root (e.g. strawberry; spider plant)
5. bulb & corm - similar: very small, knotty stem with either fleshy or papery stems (bulb e.g. - onion; corm e.g. - look one up! ha!)
6. cladophyll - flattened stem that often serves the photosynthetic function of a leaf. (e.g. - Earleaf acacia; cactus) (Note: cactus spines are actually leaves!)
Let's have a look at the general anatomy of THE LEAF
cuticle (secreted by the epidermis and consisting of wax)
epidermis
palisade mesophyll
spongy mesophyll
vein system (xylem on top; phloem on bottom)
More about leaves...
leaf arrangement:
1. opposite
2. alternate
3. whorled
leaf shape:
1. simple - single blade
2. compound - divided blade
leaf shape, continued:
leaves that are simple or compound may also be:
pinnate (feather-shaped), palmate (palm-shaped), ovate (round)
cordate (heart-shaped), etc. (there are dozens of terms, but I will not torture you with this. I will torture you with other things.)
(See figure 7.3 of your text to study these types, arrangements and shapes of leaves)
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Leaves have evolved to best support the life of the plant and can be related to the type of habitat in which the plant lives:
1. xeriphyte - a plant that lives in a very dry, arid habitat, and has special features which allow it to conserve water.
e.g. - cactus, most desert plants; in southern Florida: slash pine, coontie, many pineland species.
2. hydrophyte - plant that is evolved to live in a very wet environment.
e.g. - water lilies; mangroves; many wetland species
3. mesophyte - plant adapted to live in moderately moist environment.
NOTE: Most plants cannot be neatly ascribed to any one of these categories; they exist along a continuum, and may "lean" one way or the other.
