Plant Morphology: Observations on Salvia splendens

This is a paper based on observations I made on the plants I planted at the beginning of the semester (Fall 2001)! Let me say that I'm SO PROUD of my plants!

I took the time to fix the footnotes onto this webpage for this paper since I do some observing down there, too. I guess now, they'd be called endnotes.

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Observations on
Salvia splendens
of the Lamiaceae family

Plant Biology 107 --- Prof. D.R. Kaplan
Fall 2001

In its full-grown glory, Salvia splendens arrests the eyes with a striking profusion of little red flowers towering over a mop of pointy triangle leaves. This plant is commonly known as Scarlet Sage or Red Blazer and is a member of the Lamiaceae (Mint) family. While the plant's mature figure is a ravishing tribute to nature's majesty, this form is only a fragment of the total story. The plant reveals more of its story upon organized examination of its morphology --- traced throughout its development --- starting from its inception as a seed.

SEED and SEEDLING
A typical S. splendens seed measures 3 mm long. The emergent dicotyledonous seedling is epigeal and phanerocotylar. They first break ground approximately two to three weeks after planting.

ROOT CHARACTERISTICS
The allorhizic root system is oriented around a main taproot. When the seedling breaks ground, there's already a visible distinction between the hypocotyl of the little shoot and the little taproot. As the plant grows, lateral roots branch off from the main taproot and fan out profusely in order to absorb water and nutrients for the plant. This profuse root system establishes a secure, penetrative grip to the ground as well.

PHYLLOTAXIS and SHOOT CHARACTERISTICS
S. splendens is a dicotyledonous angiosperm that exhibits an opposite and decussate phyllotaxis during the vegetative juvenile phase. Its shoot exhibits the 'square stem' property characteristic of members of the Lamiaceae family. All branching is acrogenous --- in the form of axillary branches, from buds originally produced at the meristem. Both the main axis and these axillary branches exhibit the square stem property, supporting the concept of the re-iterative nature of lateral branches relative to the main shoot.

True to its opposite and decussate nature, the shoot of S. splendens exhibits bilateral symmetry --- even in the axillary branching pattern at each node. The axillary buds of each opposite leaf pair develop simultaneously so that the main shoot remains bilaterally symmetrical. Furthermore, owing that each of these axillary branches is an image of the main shoot, they are each bilaterally symmetrical, with opposite and decussate phyllotaxis.

Internodal width along the shoot (vegetative through florescence --- more concerning S. splendens's heteroblastic nature later in the paper) follows a gradually decreasing Erstarkungswachstum rhythm. The shoot is thickest at the basal end and thinnest at the proximal florescent end.

Internodal elongation along the shoot follows a rhythmic pattern as well, see Plot 1. At the change of phase, there is a spurt in internodal elongation between the node of the last vegetative leaf pair and the first flowering node. This internode generally has twice the length of the last vegetative internode below it, which probably serves to set the inflorescence at a more marketable position for attracting pollinators. Internodal elongation for the inflorescence part of the shoot follows a different rhythmic pattern than the vegetative part of the shoot, but there is a definite rhythm. While the vegetative internode lengths gradually increase then decrease along the shoot, the inflorescence internode lengths start big and gradually decrease (see endnote 1). The inflorescence shoot can be characterized as a raceme because of the internodal elongation.

LEAF MORPHOLOGY
The leaf blades of S. splendens are simple, normally bifacial, and symmetrically triangular with crenated margins. The leaf blades are flattened in the median plane, relative to the shoot's axis, and are epeltate. The adaxial side of the leaf blades are dark green and are the main photosynthetic surfaces of S. splendens. Young leaves coil adaxially, indicating that the adaxial, photosynthetic side initially grows faster than the abaxial side. As the leaf blades broaden, the initial coiling flattens out.

There is intercalary distribution of growth in the leaf blade regions. Young leaves have a long length axis and a narrow width axis. Given time, each of the vegetative leaves significantly widen at the base to become either ovate or triangular, depending upon its insertion along the axis. However, the tips of each successive vegetative leaf develop less and less width relative to the blade-widening at the basal part of the leaf. This leads to a pointy, narrow shape at the distal ends (tips) of the leaves where the blade has remained narrow while the rest of the blade has widened. See Figure 1.

An interesting note: corresponding to this above, the margins of successive vegetative leaves become less crenate and more dentate. This creates a spectrum of leaf margins corresponding to overall leaf shape. The oldest vegetative leaves have round crenate margins, a rounded tip, and an overall ovate shape. Each successive developed vegetative leaf has sharper crenate margins, a more defined 'pointy' tip, and less widening at the basal end which leads to an overall shape that tends toward triangular.

The petioles of the leaves are also bifacial. The adaxial side is evident as a depressed line running the length of the petiole that smoothly joins with the adaxial side of the leaf blade. The abaxial side is radially round (or rather, it is not dorsiventrally flattened --- think: the petiole would be radially symmetric if it were unifacial) and joins the leaf blade as a midrib that runs all the way to the distal end of the blade. In older and larger vegetative leaves, both the petiole and the midrib develop to be thick and strong. They serve to support the leaf blade in the median plane.

Comprehensively, the holistic shape and structure of the leaf make mechanical sense. For physical and biological reasons, the leaf blade develops wide to create a large surface area with which to collect sunlight, and the petiole and midrib develop thickly to support the expanded leaf blade. For maximal mechanical equilibrium, the leaf should be most massive (i.e. widest) where it can receive the most mechanical support (see endnote 2). S. splendens's leaf morphology maximizes all of these factors so that the whole leaf unit can maximally perform a function of receiving sunlight.

HETEROBLASTIC
In the transition from the juvenile vegetative phase to the adult reproductive phase along any particular axis, leaves along said axis display serial homology, and there is rhythmic change in the axis's internodal lengths, which was displayed in Figure 1. The shoot is heteroblastic because it is morphologically different between its juvenile and adult phases. The shoot itself does not change in habit; it remains orthotropic during both phases while the apical meristem transforms from producing vegetative leaves to producing inflorescence bracts and flowers.

The first vegetative leaves are the simplest and most ovate, as was previously described. The leaf blade lengths, blade widths, and petioles develop differentially, as was displayed in Figure 1. As the shoot nears its transition to a flowering shoot, the leaves reduce in size (in length, width, and petiole) at successive internodes. Note that although overall leaf size is now diminishing along the shoot, the 'pointy tip' property previously described becomes more marked --- these leaves are thus obviously from an older age of the apical meristem. In the inflorescence, bracts are the scarlet color of the calyx and corolla. The bracts develop precociously and abscise soon after internodal elongation. The first bracts sometimes exhibit the intermediate feature of being partly green.

In addition, the coloring of the shoot sometimes changes from pale green (vegetative region) to reddish (reproductive region), duplicating the color of the flowers. This may serve to help attract pollinators or may simply be an artifact of the shoot's new inflorescence nature.

FLOWERS
S. splendens's euanthic flowers are a bright scarlet. The synsepalous calyx and the sympetalous corolla are the most conspicuous scarlet parts of the flower. During the inflorescence shoot's internodal elongation, the node's respective calyx develops precociously. It is initially fused together to enclose the rest of the flower.

The filaments of the stamens and the style of the carpel are fused to the corolla. There are two stamens and each consists of one filament with a fertile theca at one end and a sterile theca at the other. The carpel's superior and perigynous ovary is fused to the calyx. See Figure 2. The corolla, along with the fused stamens and style, readily abscises soon after emerging from the opened calyx (leaving only a short window of opportunity for pollinators to come pollinate the flower). The calyx remains to protect the ovaries while the fertilized ovaries develop.

TROLL'S SHOOT AXIS ZONATION APPLIED to S. SPLENDENS
Troll categorized plant inflorescences with respect to the whole plant vegetative body and marked out general zones along the shoot axis which correspond to certain morphological plant potentials and characteristics. In context of Troll's zonation methodology: S. splendens has no zone of innovation because it is an annual. S. splendens has vegetative axillary branching in the zone of inhibition. S. splendens is capable of many axillary inflorescence shoots (this is visually apparent in the observed population), which defines the enrichment zone. S. splendens exhibits a main axis florescence. Whether S. splendens has a polytelic (open) or monotelic (closed) inflorescence nature remains undetermined because the observed population of plants have not grown to the age where this becomes apparent (the shoot apical meristems are still producing inflorescences).

INTERESTING OBSERVATIONS on UNCONTROLLED PLANTS
Very early in one seedling's growth (3 weeks), bugs ate away the first vegetative leaves and the meristem. I was, naturally, upset, but this turned out to be a boon because the next week I witnessed precocious axillary branching. Because the shoot was reduced to the pair of cotyledons and stubble, a 'new' main axis emerged from the more intact axil of the two leaves.

In an unrelated matter, I planted a new tray of seeds three weeks after the first tray. By the time these seedlings emerged, they were growing in the shadow of the older plants. And by the time these seedlings reached the age when the original tray of seedlings began axillary branching (six weeks), they were really in the shadow of the older plants who had grown big. The shoots in the new tray exhibited profuse axillary branching, with axillary buds growing into shoots from nearly every axil --- much more than the with the original tray of plants. This profuse adventitious axillary branching may be due to lack of sunlight: the shadowed plant decides it needs more photosynthetic surface area with which to collect sunlight and initiates more axillary branching than is typical for S. splendens.

ENDNOTES
(1) SIDE NOTE: This makes mechanical sense because in order for the thinner shoot axis to support the mass of the flowers along such an extended length, something must compensate for the axis to stay balanced in equilibrium. By doing this, the plant has accounted for the contingency of when the older, lower flowers have fallen off and the thin shoot axis must support the flowers left at the top (i.e. when the center of mass has changed). Clever plant!
(2) SIDE NOTE: I notice that in the largest, broadest, and most developed leaves, the leaf blade grows beyond triangular at the basal end and basally extends into a proto-oval shape. This moves the center of mass of the leaf blade more toward the center of the leaf (so that the leaf is not 'bottom-heavy'). In a nutshell, this maximizes the efficiency of the distribution of weight of the leaf blade in order for the petiole and midrib to maximally support it using minimal effort. Amazing!

REFERENCES
Kaplan, D. R. Principle of Plant Morphology. Course and Laboratory Manual. USA: Odin Readers, 2001.
Kaplan, D. R. Principle of Plant Morphology. Vols I-IV. USA: Odin Readers, 2001.
Steeves, Taylor A. and Ian M. Sussex. Patterns in Plant Development. 2nd ed. NY, USA: Cambridge University Press, 1989.

CAPTIONS

Plot 1. Internode lengths along the main axis of one S. splendens plant (80 days old). Notice the rhythmic uni-modal (bell-curve) trend typical of opposite and decussate angiosperms in the vegetative phase (internodes 1 through 7), such as Coleus. Also notice the spurt in internodal elongation between the node of the last vegetative leaf pair and the first flowering node (internode number 8), which is typical to this population of S. splendens plants. Furthermore, notice the declining trend in internodal elongation along the reproductive portion of the main axis shoot (internodes 9 through 21).

Figure 1 (next two pages). Specimens of leaves along the main axis of one opposite and decussate S. splendens plant (60 days old). Each successive leaf is one leaf of the leaf pair at a node. The first specimen is the shoot's first vegetative leaf, the last green leaf is the last vegetative leaf before the main shoot axis enters its reproductive phase, and the following leaves are bracts. Particular notes of interest: 1) Differential growth between the lengths and widths of the leaf blade, especially the marked lack of blade-tip widening in successive leaves, corresponding to a later insertion along the shoot axis. Marked 'pointy tip' is also evident in successive bracts. 2) Differential growth between the leaf blade and petiole. 3) Definition of leaf blade margins: a spectrum from crenate to dentate. 4) Petiole and midrib thickness relative to leaf blade size (visible from rear). 5) Leaf size, morphology of leaf blade, and leaf blade's respective center of mass (see footnote 2).

Figure 2. S. splendens flower. Left, flower parts left to right: synsepalous calyx, sympetalous corolla, anthers and fertile thecas, style and stigma. Right, longisection of the sympetalous corolla, showing: 1) the stamen filaments which are fused to the corolla, 2) the fertile and sterile theca of the stamen, and 3) the style and stigma.

Let's return to Pieces of Work!