Chapter 9: Flowers and Reproduction
Types of Reproduction
Asexual Reproduction
within Angiosperms the most common method of asexual reproduction is Fragmentation
a large spreading or vining plant grows several meters in length
the individual parts become self-sufficient by establishing adventitious roots
a parent organism breaks into fragments, each capable of growing independently into a new organism
Sexual Reproduction
involves flowers which produce the necessary structures and cells
The Plant Life Cycle
diploid adults have sex organs that produce haploid sex cells called gametes
either sperm or eggs
by meiosis
one sperm and one egg are brought together forming a new single diploid cell called a zygote
the plants life cycle is more complex
the plants you are familiar with are all apart of one phase called the sporophyte phase/ generation
sporophytes are always diploid
have organs that are capable of undergoing meiosis
in plants meiosis results in spores
the difference between spores and gametes is huge
gametes can fuse with each other in a process called syngamy
plant spores can't undergo syngamy
they can undergo mitosis and grow into a entire new haploid plant called a gametophyte
there are two type of mammalian gametes
small sperm (microgametes) that can swim
large eggs (megagametes) that don't swim
this is also true for many plants and its known as oogamy
In plants the 2 types or gametophytes have grown from 2 types of spores
having 2 types of spores is known as heterospory
Flower Structure
a stem with leaf-like structures
flowers never become woody; secondary growth doesn't occur in plants
The flowers stalk is called a pedicel
at the other end where the other flower parts attach is the receptacle
there are 4 types of floral appendages
sepals
petals
carpels
stamens
Most flowers that have all 4 are known as complete flowers
Lacking one or more of the 4 is called an incomplete flower
Sepals
lowermost and outermost of the four floral appendages
they are modified leaves that surround and enclose the other flower parts as they mature
thickest, toughest and waxiest of the flower parts
keep bacterial and fungal spores away
maintain high humidity within the bud
all sepals together are referred to as calyx
Petals
above the sepals on the receptacle are petals
together make up the corolla
sepals and petals together constitute the flower's perianth
are also leaf-like, being broad, flat and thin
differ from leaves as they contain pigments other than chlorophyll
each plant species has flowers of distinctive size, shape, color, and arrangement of petals allowing pollinators to recognize specific species
Stamens
above the petals are stamens
collectively known as the androecium
referred to as the male part of the flower
they have two parts
filament (its stalk)
the anther
where pollen is actually produced
composed of diploid cells
four large columns of tissues become distinct as some cells enlarge and prepare for meiosis
the microsporocytes continue to enlarge and then undergo meiosis each producing 4 microspores
neighboring anther cells, in a layer called the tapetum
act as nurse cells
contribute to microspore development and maturation
Carpels
constitute the gynoecium
located at the highest level on the receptacle
have three main parts
stigma
collects pollen grains
style
elevates the stigma to useful position
ovary
where megaspores are produced
a flower can have zero to many carpels
usually fused together into a single compound structure
called a pistil
inside the ovary are placentae
regions of tissue that bear small structure called ovules
ovules have a short stalked called a funiculus
has a central mass of parenchyma called the nucellus
Gametophytes
Microgametophytes
very small and simple, consisting of at most 3 cells located within the original pollen cell wall
the microspore nucleus migrates to the side of the pollen grain and lies near the wall
there it divides mitotically, producing a large vegetative cell and a small lens shaped generative cell
then divides and creates two sperm cells
in about 30% of angiosperms species, formation of sperm cells occurs even while pollen is still located in the anther
after a pollen grain lands on the stigma, it germinates by producing a pollen tube
this penetrates into the loose, open tissue of the stigma
Megagametophytes
in one type of development, the nucleus undergoes three mitotic divisions
producing two, four and then eight haploid nuclei all in a single undivided cell
the term for a multinucleate megagametophyte is an embryo sac
the nuclei migrates through the cytoplasm, pulled by microtubules, until 3 nuclei lie at each end and two in the center
walls then form around the nuclei, and the large, eight-nucleate megaspore becomes a megagametophyte with 7 cells and one that is binucleate.
the 7 cells are one large central cell with two polar nuclei, three small antipodal cells and an egg apparatus consisting of to synergies and an egg
obtains all nutrients from parent sporophyte
Fertilization
syngamy of the sperm and egg involves both plasmogamy , fusion the protoplasts of the gametes, and karyogamy which is fusion of the nuclei
in angiosperms only, the second sperm nucleus released from the pollen tube migrates from the synergid into the central cell
it undergoes karyogamy with both polar nuclei, establishing a large endosperm nucleus
it is triploid, containing three full sets of genes
both sperm nuclei undergo fusions, one with the egg and the other with the polar nuclei
this is called double fertilization
Embryo and Seed Development
the endosperm nucleus proliferates the zygote begins to grow, always by both nuclear and cellular divisions; a co-ecyntric stage never occurs in the embryo
the zygote grows into a small cluster of cells, part of which become the embryo proper; the other part becomes a short stalk-like structure called the suspensor
this pushes the embryo deep into the endosperm
the suspensor is usually delicate and ephemeral in angiosperms
it is crushed by the later growth of the embryo and not easily detected in a mature seed
cells at one end of the suspensor continue to divide mitotically, developing into an embryo
the cells are first arranged as a small sphere, the globular stage
the end of the embryo farther from the suspensor initiates two primordial that will grow into two cotyledons in basal angiosperms and eudicots
later in a torpedo stage, the embryo is an elongated cylinder: a short axis is established, consisting of the radicle, epicotyl and hypecotyl
A mature seed in which endosperm is rather abundant is an albuminous seed
If endosperm is sparse or absent at maturity the seed is exalbuminous
Fruit Development
as the ovule develops into a seed, the ovary matures into a fruit
development varies with the nature of the carpels as well as the nature of the mature fruit
the stigma, sepals, petals and stamens will wither away, although they do persist at least temporarily
often 3 layers become distinct during growth:
the exocarp which is the outer layer, the skin or peel
the middle layer is the mesocarp or flesh
the innermost layer is the endocarp
can be tough as stones or a cherry pit or super thin
the entire fruit wall no matter the number of layers its composed of is called the pericarp
Flower Structure and Cross Pollination
Cross Pollination
is the pollination of a carpel by pollen from a different individual
Self Pollination
is pollination of a carpel by pollen from the same flower or another flower of the same species
compatibility barriers
chemical reactions between pollen and carpels that prevent pollen growth
Monoecious and Dioecious Species
Dioecy
a species might have an individual that produces only carpellate flowers
examples:
marijuana
dates
papaya
life cycle
microgametophytes
megagametophytes
staminate sporophytes
carpellate flowers
Monoecy
is a condition of having staminate flowers, located on the same plant as carpellate flowers
examples
corn
cattails
Animal Pollinated Plants
Coevolution
a flower becoming adaptive from visitation by a particular insect, and the insects for efficient exploitation of the flower
Ovary Position
Inferior Ovary
epigynous
also can result if receptacle tissue grows upward around the ovary
The most common arrangement in which no fusion to the ovary occurs and its obviously above all the other parts
superior ovary
hypogynious parts
