Lesson 8. External Plant Structure

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Stems

Twig

Terminal bud and lenticels on Pop Ash (Fraxinus carliniana), Florida.
Terminal bud and lenticels on Pop Ash (Fraxinus caroliniana), Florida.

 

Lateral buds (axillary buds) and lenticels on Pop Ash.   The leaves have fallen, with leaf scars in side view just below (to the left of) the buds.
Lateral buds (axillary buds)  on pop ash. The leaves have fallen, with leaf scars  just below (to the left of) the buds.  Lenticels are conspicuous.

 

Axillary thorn (axillary bud grew into a thorn) on citrus
Axillary thorn (axillary bud grew into a thorn) on citrus

 

Brussels sprouts are giant axillary buds.

Taproots and Fibrous Roots

Taproot on Eriocaulon decangulare, a Monocot (by John Bradford)
Fibrous roots on Eriocaulon decangulare, a Monocot (by John Bradford)
Taproot on Pouzolzia zeylanica, a Dicot.
Taproot on pouzol weed (Pouzolzia zeylanica), a Dicot.

Specialized stem types

A bulb is a thin stem surrounded by layered thick leaf bases, as in an onion.

A corm is a short thick stem at ground level.  It looks like a bulb but is all stem.

A rhizome is a horizontal stem, usually underground.  It looks like a root but has stem structure, including axillary buds.

A stolon is an above-ground rhizome, or “runner.”

A tuber is an extra-thick underground stem.

Stipules

Stipules. Stipules are usually paired as you see the two pointed objects at the center.  The (compound) leaf with 4 leaflets visible  is bent to the left, and the stem is tilted to the right.  The axillary bud is tiny and hidden between the two stipules.
Stipules are usually paired,  as are the two long pointed upright triangular stipules at the photo center. The (compound) leaf with 4 leaflets visible is bent to the left, and the stem is tilted to the right. The axillary bud is hidden between the two stipules.  The petiole to the left has an extrafloral nectary on it.
Stipule.  Many plants have stipules, variable small outgrowths  where the petiole joins the stem.  Stipules are usually but not always paired, with one on each side of the petiole base.  Here are two stipules on the base of a compound rose leaf. They are fused to the petiole.
Two two stipules on the base of a rose leaf,  fused to the petiole.
sometimes stipules form a fringe (on Mexican-clover, Richardia grandiflora).
Sometimes stipules form a fringe (on Mexican-clover, Richardia grandiflora).  Incidentally, nice example of hispid pubescence. The elliptic green growth tilted left behind the fringed stipule is an axillary bud beginning to grow out.
Stipule.  Members of hte Coffee Family have one stipule between the opposite petioles, as on this firebush (Hamelia patens).
Members of the coffee family have one stipule between the opposite petioles, as on this firebush (Hamelia patens).  The stipule is the tail-shaped redish organ rising between the two petioles.
Stipule. some plants (red mangrove, magnolias, figs) have a single stipule that forms a dunce-cap cover over the apical bud.  This is strangler fig (Ficus aurea).
Some plants (red mangrove, magnolias, figs) have a single stipule formed into a dunce-cap cover over the apical bud. This is strangler fig (Ficus aurea).
Stipules.  These stipules are paired in the traditional fashion, one on either side of the leaf scar.   They  are modified as thorns in this are white candelabra-cactus (Euphorbia lactea).   This is not a true cactus, but rather a member of the spurge family.
These stipules are paired, one on either side of the light tan leaf scar. They are modified as thorns in this candelabra-cactus (Euphorbia lactea).  See further commentary below in the caption of the cactus areole.
Ouch!  Paired stipules pierce flesh.
Ouch! Paired stipules attack.
In a true cactus such as htis, the spine cluster (areole) is the axillary bud.  The lowermost spine is the leaf whose axillary bud is the areole.
In a true cactus such as this, the spine cluster (areole) is the axillary bud. The lowermost spine is the leaf whose axillary bud is the areole.  The candelabra cactus above is not a true cactus, as evidenced by its stipular spines instead of the cactus areole.

Pubescence (= hairs)

Glabrous = hairless
Glabrous = hairless.  Incidentally, these Nasurtium leaves are peltate.
Tomentose = fuzzy. (Rubus sp.)
Tomentose = fuzzy, like felt (Rubus sp.)
Hispid = bristly.  See also the hispid fringed stipule above.
Hispid = bristly

 

Compound and lobed leaves

A compound leaf looks like a stem with leaves on it,  but the axillary bud is at the base of the entire unit.   The “leaves” on a compound leaf are leaflets with no axillary buds of their own.  A pinnately compound leaf has the leaflets along a stalk (rachis).  A palmately compound leaf has the leaflets radiating.  If a leaf is not cut all the way into separate leaflets, it is pinnately or palmately lobed, that is, there is a web of leaf tissue between the lobes.  Click on the thumbnails to enlarge them.

Trifoliate leaf.  A trifoliate leaf has three leaflets.
Trifoliate leaf. A trifoliate leaf has three leaflets.
Virginia creeper has palmately compound leaves.  At the end of the branch, note the tendrils.    Tendriles are stingy, twisty organs to allow some vines to grab hold.
Virginia creeper (Parthenocissus quinquefolia) has palmately compound leaves. At the end of the branch, note the tendrils. Tendrils are stingy, twisty organs to allow some vines to grab hold.

 

Virginia creeper (Parthenocissus quinquefolia) clinging to concrete wall
Virginia creeper  clinging to concrete wall
Virginia creeper has beautiful tendrils
Virginia creeper has beautiful tendrils
Virginia creeper tendrils end in sticky pads.  The glue is a mix of pectins which becomes lignified.
Virginia creeper tendrils end in sticky pads. The glue is a mix of pectins which becomes lignified.
Tendrils on Smilax
Tendrils on Smilax

Leaf Arrangements

One leaf per node = alternate

Two leaves per node = opposite

Three or more = whorled

Leaf Shapes

Leaf shapes colored stroked lettered

Tips and Margins b&w background colored in

Cordate is heart-shaped.
Cordate is heart-shaped.  The margins are entire on this glabrous leaf.
Sagittate is shaped like an arrowhead.
Sagittate is shaped like an arrowhead.
Lanceolate. Shaped like a lance tip, a candle flame
Lanceolate. Shaped like a lance tip, a candle flame.  A long tapered tip is acuminate (ah-CUUM-ah-nate).
Oblanceolate is  upside-down lanceloate, widest above the middle.
Oblanceolate is upside-down lanceloate, widest above the middle.
Ovate is similar to lanceolate but broader, egg-shaped.  These opposite ovate leaves have serrate margins.
Ovate is similar to lanceolate but broader, egg-shaped. These opposite ovate leaves have serrate margins.  Rough surfaces are rugose.
Obovate is upside-down ovate, widest above the middle., broader then oblanceolate.
Obovate is upside-down ovate, widest above the middle., broader than oblanceolate.
Peltate is shaped like a thumb tack, the stalk attached to the center.
Peltate is shaped like a thumb tack, the stalk attached to the center.  These leaves have a crenate (with dull rounded teeth) margin.
A serrate margin has saw-teeth (angled toward the leaf tip).
A serrate margin has saw-teeth (angled toward the leaf tip).  Note the stipules flanking the axillary bud.
A dentate margin has teeth sticking straight out.
A dentate margin has teeth sticking straight out.   The leaf tip here is obtuse (that is, a blunt angle).
A mucronate leaf tip has a tiny tooth.
A mucronate leaf tip has a tiny tooth.
An emarginate leaf tip has a notch.
An emarginate leaf tip has a notch.

Practice Quiz

1. Leaves attached two-by-two are:
A. Alternate
B. Opposite
C. Whorled
D. Paripinnate

2. A leaf margin resembling sawteeth is:
A. Serrate
B. Crenate
C. Sinuous
D. Pinnate

3. A compound leaf where the leaflets radiate like spokes is:
A. Pinnately compound
B. Decompound
C. Palmately lobed
D. Palmately compound

4. Fuzzy like felt:
A. Hispid
B. Hirsute
C. Peltate
D. Tomentose

5. Heart-shaped:
A. Hastate
B. Sagittate
C. Cordate
D. Hearty

6. Hairless:
A. Glabrous
B. Nodose
C. Nudate
D. Hirsute

7. Roots along the stem above the ground:
A. Fibrous
B. Lianate
C. Adventitious
D. Stolons

8. Shaped like a thumb tack:
A. Tacky
B. Obovate
C. Saccate
D. Peltate

9. Another name for axillary buds:
A. Lateral buds
B. Terminal buds
C. Bundle traces
D. Leaf scars

10. Immediately above every leaf attachment:
A. Terminal bud
B. Lenticel
C. Axillary bud
D. Leaf scar

11. Paired outgrowths at petiole base:
A. Lodicules
B. Elaiosomes
C. Stipules
D. Lateral buds

12. Stem between leaf attachment points:
A. Interludes
B. Intrazones
C. Alamodes
D. Internodes

13. Leaf blade with notch at tip:
A. Clavate
B. Emarginate
C. Glaucous
D. Cordate

14. Leaf blade shaped like a candle flame:
A. Obovate
B. Lanceolate
C. Oblate
D. Flameous

15. Upside-down egg, widest above middle:
A. Obovate
B. Lanceolate
C. Elliptic
D. Dendritic

16. A leaf with leaflets along what looks like a stem (but the leaflets do not have axillary buds) is:
A. Palmately compound
B. Pinnately lobed
C. Ultracompound
D. Pinnately compound

17. Describe each leaf below. shape? Margin? Tip? Is it compound?  Pubescence?

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