BOTANY LECTURE REVIEWEREVOLUTION AND DIVERSITY OF WOODY AND SEED PLANTS (CHAPTER 5) PAGES 129-162

FOR A MORE ELABORATE DISCUSSION AND ILLUSTRATIONS, PLEASE REFER TO YOUR TEXTBOOK.

LIGNOPHYTES – WOODY PLANTS (ALSO CALLED LIGNOPHYTA) . (PAGE 129)

Monophyletic lineage of euphyllous vascular plants sharing derived features:o Vascular Cambium – gives rise to woodo Cork Cambium – produces cork (Figures 5.1, 5.2 page 130, 131)

Secondary Growth – is what we call the growth of the vascular and cork cambiumo it is because it initiates after the vertical extension of stems and roots due to cell expansion

(primary growth) Vascular Cambium – is a sheath, or hollow cylinder, of cells that develops within the stems and

roots as a continuous layer, between the xylem and phloem.o Cells of the vascular cambium divide mostly tangentially (parallel to a tangential plane),

resulting initially in two concentric layers of cells (Figure 5.3A page 132)

One of these layers remains as the vascular cambium and continues to divide indefinitely

The other layer eventually differentiates into either: Secondary Xylem – if produced to the inside of the cambium Secondary Phloem – if produced to the outside (Figure 5.3A,B)

Bificial – a type of growth wherein layers of cells are produced both to the inside and outside of a continuously generated cambium.

*Note: more 2o xylem is produced than 2o phloem 2o cambium independently evolved in fossil lineage w/in the lycophytes, but this

cambium was unificial, producing 2o xylem but not 2o phloemo Secondary growth results in an increase of the width or girth of stems and roots

Occurs both by expansion of the new cells generated by the cambium and by accompanying radial divisions, increasing the number of cells within a given growth ring.

Many woody plants have regular growth periods, e.g., forming annual rings of wood (Figure 5.4 page 133)

Cork Cambium – similar to vascular cambium, only it differentiates near the periphery of the stem or root axis.

o Cork cambium and its derivatives constitute the periderm (referred to as the outer bark) Cork – the outermost layer of the periderm (Figure 5.3B page 132)

Cork cells contain a waxy polymer called suberin (similar to cutin) that is quite resistant to water loss

Secondary xylem, or wood – functions in structural support.

Cork – functions as a thick layer of cells that protects the delicate vascular cambium and secondary phloem from mechanical damage, predation, and desiccation.Dendochronology – study of the past using the features of the wood anatomy

Monopodial Growth – another feature lignophytes possess, in which a single main shoot branches from lateral (usually axillary) buds.

o It enabled woody plants in particular the capability of forming extensive (sometimes massive) woody branching systems permitting them to survive and reproduce more effectively.

SPERMATOPHYTES – SEED PLANTS (SPERMATOPHYTA) . (PAGE 131)

o Major evolutionary novelty: seed (Figure 5.5 page 133)

o Seed – an embryo – an immature diploid sporophyte developing from the zygote, surrounded by nutritive tissue and enveloped by a seed coat

Radicle – an immature root Epicotyl – a shoot apical meristem Cotyledons – one or more young seed leaves Hypocotyl – the transition region between root and stem (Figure 5.5, 5.10)

SEED EVOLUTION

Probable steps in seed evolution are as follows (Figure 5.6 page 134)

1. Heterospory – the formation of two types of haploid spores within two types of sporangia: large, fewer-numbered megaspores, which develop via meiosis in the megasporangium, and small, more numerous microspores, the product of meiosis in the microsporangium (Figures 5.6, 5.7 page 135, 135)

Homospory – ancestral condition where only a single spore type forms Megaspore – develops into a female gametophyte, bearing archegonia Microspore – develops into a male gametophyte, bearing antheridia

2. Endospory – is the complete development of, in this case, the female gametophyte within the original spore wall (Figure 5.6 page 134)

Exospory – ancestral condition where spore germinates and grows as an external gametophyte

3. Reduction of megaspore number to one. Occurred in two ways: First: the number of cells within the megasporangium that undergo meiosis (each termed

a megasporocyte or megaspore mother cell) was reduced, from several to one. This single diploid megasporocyte gives rise to four haploid megaspores

Second: of the four haploid megaspores produced by meiosis, three consistently abort, leaving only one functional megaspore. This single megaspore also undergoes a great increase in size, correlated with the increased availability of space and resources in the megasporangium.

4. Retention of the megaspore. Instead of the megaspore being released from the sporangium (the ancestral condition), in seed plants it is retained within the megasporangium. This was accompanied by a reduction in thickness of the megaspore wall.

5. Evolution of the integument & micropyle. The final event in seed evolution was the envelopment of the megasporangium by a layer of tissue, called the integument.

The integument grows from the base of the megasporangium (which is often called a nucellus when surrounded by an integument) and envelopes it, except at the distal end.

Fossil evidence suggest that the integument likely evolved from telomes (ancestral branches) that surrounded the megasporangium

These “preovules”, i.e., ovules prior to the evolution of integuments possessed a rim or ring of tissue at the apex of the megasporangium, the lagenostome, which functioned to funnel pollen grains to a pollination chamber

Epitome of seed evolution = “fusion” of the telomes to form the integument–a continuous sheath that completely surrounds the nucellus.

Micropyle – a small pore at the distal end in the integument of all extant seed plants.

– functions as the site of entry of pollen grains (or in angiosperms, of pollen tubes). – also functions in mechanics of pollination droplet formation and resorption.

POLLINATION DROPLET (PAGE 135)

One possible novelty associated with seed evolution A droplet of liquid secreted by the young ovule through the micropyle (Figures 5.10A, 5.17I)

Is mostly water plus some sugars or amino acids and is formed by the breakdown of cells at the distal end of the megasporangium (nucellus)

Pollination chamber – cavity formed by this breakdown of cells Functions in transporting pollen grains through the micropyle.

o Occurs by resorption of the droplet, which “pulls” pollen grains that have contacted the droplet into the pollination chamber

(kung baga yung mga nadikit na pollen eh hinigop kasama yung droplet)

POLLEN GRAINS (PAGE 135)

Concomitant with the evolution of the seed was the evolution of pollen grains (Figure 5.8)

Pollen grains – an immature, endosporic male gametophyte– extremely reduced male gametophytes, consisting only of a few cells

Endospory in pollen grain evolution was similar to the same process of seed evolution, involving the development of the male gametophyte within the original spore wall.

They are called “immature” male gametophytes because, at the time of their release, they have not fully differentiated.

Released from microsporangium → transported to the micropyle of the ovule (or, in angiosperms, to the stigmatic tissue of the carpel) in order to ultimately effect fertilization → male gametophyte completes development by mitotic divisions and differentiation → grows an exosporic pollen tube ,

which functions as an haustorial organ, obtaining nutrition by absorption from the surrounding sporophytic tissue

POLLEN TUBE (PAGE 136) (SEE FIGURE 5.9 PAGE 136)

Male gametophytes of all extant seed plants form a pollen tube soon after the pollen grain make contact with the megasporangial (nucellar) tissue of the ovule.

In extant seed plants the ancestral type of pollen tube (found in cycads and ginkgophytes) was haustorial, in which the male gametophyte feeds (like a parasite) off the tissues of the nucellus

Motile sperm is delivered from this male gametophyte into a fertilization chamber, where the sperm swims to the archegonium containing the egg, a process known as zooidogamy (zooin, animal + gamos, marriage)

In conifers (including Gnetales), pollen tubes are also haustorial, but deliver nonmotile sperm cells to the archegonium or egg, a process known as siphonogamy (siphon, tube + gamos, marraige)

OVULE AND SEED DEVELOPMENT (PAGE 136)

After pollination, the megasporocyte develops within the megasporangium of the ovule (Figures 5.10A, 5.11A page

137, 138)

o The megasporocyte is a single cell that undergoes meiosis, producing a tetrad of four haploid megaspores, which in most extant seed plants are arranged in a straight line, or linearly (Figure 5.10A

page 137)

o The three megaspores that are distal (away from ovule base) abort; only the proximal megaspore (near the ovule base) continues to develop.

o In the pollination chamber, the resorbed pollen grains (Figures 5.10A, 5.11A) develop into mature male gametophytes and form pollen tubes, which grow into the tissue of the megasporangium (Figures

5.10A, 5.11B).

o The functional megaspore greatly expands, accompanied by mitotic divisions, to form the endosporic female gametophyte (Figure 5.10A, 5.11B,C)

o In the seeds of gymnosperms, archegonia differentiate at the apex of the female gametophyte

(Figure 5.11C,D)

o As in large egg cell and a short line of neck cells (plus typically a ventral canal cell or nucleus). Eventually, the male gametophytes either release motile sperm cells (in cycads and Ginkgo) into a cavity between the megasporangium and female gametophyte (known as the archegonial chamber; Figure 5.10A), or the pollen tube of the male gametophyte delivers sperm cells directly into the archegonial neck (in conifers).

o The end result is that a sperm cell from the male gametophyte fertilizes the egg of the female gametophyte.

o A long period of time may ensue between pollination and fertilization (Note: this is not true for the flowering

plants, in which fertilization occurs soon after pollination)

Pollination – delivery of the pollen grains to the ovule Fertilization – actual union of sperm and egg

o The resulting diploid zygote, once formed, undergoes mitotic divisions and differentiation, eventually maturing into the embryo, the immature sporophyte (Figures 5.10B, 5.11E)

o The tissue of the female gametophyte continues to surround the embryo (Figure 5.11E) and serves as nutritive tissue for the embryo upon seed germination (except in flowering plants)

o The megasporangium (nucellus) eventually degenerates.o The integument matures into a peripheral seed coat, which may differentiate into various hard

and/or fleshy layers.

SEED ADAPTATIONS (PAGE 139)

Seed provides protection, mostly by means of the seed coat, from mechanical damage, desiccation, and often predation.

Seed functions as the dispersal unit of sexual reproduction.o In many plants the seed has become specially modified for dispersal. For example, a fleshy outer

seed coat layer may function to aid in animal dispersal. In fact, in some plants the seeds are eaten by animals, the outer fleshy layer is digested, and the remainder of the seed (including the embryo protected by an inner, hard seed coat layer) passes harmlessly through the gut of the animal, ready to germinate with a built-in supply of fertilizer. In other plants, differentiation of the seed coat into one or more wings functions in seed dispersal by wind.

Seed coat may function in dormancy mechanisms that ensure germination of the seed only under ideal conditions of temperature, sunlight, or moisture.

Upon germination, the nutritive tissue surrounding the embryo provides energy for the young seedling, aiding in successful establishment.

o Interestingly, in seed plants the female gametophyte (which develops within the megaspore) remains attached to and nutritionally dependent upon the sporophyte. This is exactly the reverse condition as is found in the bryophytes.

EUSTELE (PAGE 139) (SEE FIGURE 5.12)

An apomorphy for spermatophytes. A primary stem vasculature (“primary” meaning prior to any secondary growth) that consists of a single

ring of discrete vascular bundles.o Each vascular bundle contains an internal strand of xylem and an external strand of phloem that

are radially oriented, i.e., positioned along a radius The protoxylem of the vascular bundles of a eustele is endarch in position, i.e., toward the center of the

stem.