Lecture 22

Flowers – Structure & Development

I.  Introduction

The homology of flowers with vegetative shoots dates back to concepts by Gothe.  The floral parts (sepals, petals, stamens and carpels) were homologized with leaves (phylomes, megaphylls).  This concept is supported by comparative anatomy and genetic evidence. Mutations in flowers give clues of the homology with vegetative shoots. Example 1: Cuscuta epithymum (dodder) flowers with stems growing out of gynoecium.  Example 2: Prunus flowers with leaf growing from carpel.

II.  Structure

A.  Basic parts of the flower
1. Basic terminology on floral morphology1, 2.
2.  The order of the floral whorls remains the same in virtually all angiosperms: CA, CO, A, G.  This can be seen in a L.S. of the Magnolia flower.  The one exception to this rule is Lacandonia (a monocot from Mexico in family Triuridaceae) which has CA, CO, G, A (photo, SEM photos).  A natural homeotic mutant!
3.  Variation in flowers comes:
a.  presence / absence of parts
b.  fusion of parts
4.  Summary of unspecialized (less advanced) and specialized (more advanced) floral features:
Feature Less Specialized Plants More Specialized Plants
Number of parts / whorl  many few
Arrangement of parts spiral or helical  whorled
Fusion of like parts not fused (part free) fused (parts connate)
Fusion of unlike parts rare common (adnation)
Symmetry radial (actinomorphic) bilateral (zygomorphic)
Ovary position  superior (hypogynous)   inferior (epigynous)
Flower sex  bisexual unisexual (carpellate, staminate)
        
5.  Inflorescences are modified branch systems specialized for reproduction. Often have bracts.
6. Pollination – two major categories: abiotic (wind, water) and biotic (insects, birds, mammals).  These selective agents have shaped the varied forms of flowers present today.  Various Pollination Syndromes have been described.

B. Sepals (calyx) and Petals (corolla)
1.  Resemble leaves in basic anatomy (Figure 20.1).
2.  Petals often contain crystal cells, laticifers, tannin cells and idioblasts.  Pigments in chromoplasts (e.g. carotenoids) and in sap (flavonoids such as anthocyanins). 
3.  Floral guides may be present, such as in Rudbeckia.  Petals with flavonol glycosides that absorb UV light – seen as dark by insects, guides them to part of flower.
4.  Petals epidermis may have volatile oils which gives the fragrance.  Surface epidermal cells may be convex or papillate (Figure 20.1A).

C.  Stamens (androecium)
1.  Stamens consists of filament and anther sac.  Anther is divided into two pollen sacs called thecae.  Each theca is composed of two microsporangia that produce the pollen. This is the typical tetrasporangiate type of anther.  Example: Tecomaria or Viola. Lilium anther in X.S.  Other anthers have two sporangia, are called bisporangiate (or monothecal). 
2.  Most (95%) stamens have one vein.  Some magnoliid angiosperms (such as Austrobaileya) have three veins, with the microsporangia between the central and lateral veins on the adaxial surface. These types of leaf-like stamens are called laminar stamens and they are present in many magnoliid plants such as Nymphaea, Magnolia, and Asimina.
3.  The subepidermal wall of the anther forms an endothecium (Figure 20.2 D, E, F).  Strips along the cell wall surface help with anther dehiscence (opening, releasing the pollen) – differentially shrink upon drying.

D.  Carpels (gynoecium)
1. Basic units are the carpels. May be apocarpous (such as Drimys) or syncarpous (Asarum, ovary X.S.).
2.  Carpels are conduplicate, i.e. folded in half (Figure 20.3).  Diagram.
a. Single carpel with laminar placentation, median and lateral vascular bundles, ovules arising from the midpoint. 
b. A more advanced form is marginal placentation (Figure 20.3 C). Here the carpel wall is involuted, portion of the outer surface rolled inward, becomes adjacent to other edge of carpel. The ovules are borne at the very edge (margin) of the carpel.
c. For a compound gynoecium of two fused carpels (Figure 20.3 D), there are two median vascular bundles and (only) two lateral ones (because of fusion). The locule is traversed by a septum formed from the margins of the carpels.
3. Stigma, style and ovary may or may not be differentiated in various gynoecia.
4.  Ovary – the swollen basal part of the gynoecium (either apocarpous carpel or a syncarpous condition) that bears the ovules.  Placenta = the tissue bearing the ovules.  Can be small or large.  The arrangement of ovules on the placenta is called placentation and it occurs as various types (Figure 20.4). See examples on Floral Morphology2 page.
5. Carpel vasculature. Usually 3 veins / carpel, 1 median (dorsal) and 2 lateral (ventral). The ovules are supplied from the lateral bundles.  For axile placentation (Figure 20.5 B), the lateral bundles are in the inside with phloem in, xylem outwards.
6.  Epigynous flower (Figure 20.5 C, D).  Note that this flower (of Ribes) is also perigynous, which means it has a hypanthium. The hypanthium is a cuplike structure that is composed of tissue derived (through adnation) from the CA, CO and A. Examples: Rosa, Prunus, Escholtzia, Fuchsia. Here the outer walls of the ovary are composed of fused tissue from CA, CO, A and G. 
7.  Style and stigma (image of Vernonia).  Style derived from tissue of all carpels when in the syncarpous condition (Figure 20.5 A) or one carpel (Figure 20.6 F).  Some styles are hollow (as in Lilium) and some solid.  Stigmas can be of the wet or dry type.  Surface may be papillate (Figure 20.8).  Transmitting tissue present in stigma and style leads to the ovules – nourishes the pollen tubes as they grow downward to the ovules in the ovary.  Wet stigmas are structurally and functionally like nectaries.  The liquid contains little sugar intially but lipids and phenolic compounds such as glycosides and esters that are hydrolyzed to sugars used by pollen for germination.

E.  Vascular System
1.  May be more conservative evolutionarily than other parts of flower, hence is used to interpret floral evolution (in combination with other floral characters).
2. Following the vascular traces in flowers is difficult because the axis is short and the traces branch and anastomose (join) with others.
3.  Common patterns in flowers: sepals have same number of traces as leaves, petals have one trace, stamens one trace (exceptions mentioned above for magnoliids), and carpels have 3 traces that are often branched.
4.  Vascular tissues in some flowers with inferior ovaries (epigynous flowers) is inverted towards the inside.  This indicates the ovary has developmentally sunken into the receptacle = receptacular.  Example: Opuntia cactus. Flower L.S. and diagram in Figure 20.9. Other epigynous flowers that lack inverted vascular bundles are interpreted as being axial, i.e. resulting from the fusion of CA, CO, A, and G. Example: Cornus. For flowers like Portulaca, one can see the ovary is partially fused to the other floral whorls below but still free above (termed half inferior).

III.  Development

A.  Transition from a vegetative to a floral meristem, from indeterminate to determinate growth.  Axis may form a single flower or an inflorescence (same processes for both).

B.  Induction of flowering.  Day length (photoperiodism - Wiki page) is critical.  Three types:
1.  short night [long day] plants. Period of darkness is measured, not light, hence short day / long day plants is a misnomer
2.  long night [short day] plants. 
3.  day neutral plants
4.  Processes are mediated by phytochrome which responds to red and far red light (a molecular switch - diagram).

C.  When vegetative meristem changes to a floral meristem, it becomes broad, with a mantle of meristematic cells over a core of ground tissue that is no longer growing upward.  A tunica / corpus organization may or may not be present. Figure 20.10 shows development of the floral meristem in Antirrhinum (snapdragon).

D.  Origin and development of floral organs.
1.  Histogenesis.  The floral organs arise like leaves on vegetative apices, with periclinal cell divisions beneath the protoderm.
2.  Sepals, petals and stamens look much like leaves but carpels are a bit different.  Their appearance early on depends upon whether the gynoecium is apocarpous or syncarpous. For the latter, the carpels are congenitally fused (fused from the very beginning).
3.  Development of the apocarpous carpel in Drimys (Figure 20.12).
4.  Development of the flower in Butomus (Figure 20.13)
5.  Flower development in grasses (we won't cover here).
Last updated: 14-Oct-22 / dln