a) Sepals cover the flower structure while the flower is developing. In some species these are modified to ' petals'.
b) Petals surround the male and female flower parts. Function is to attract animal pollinators.
c) Stigma is the surface on which pollen lands and the pollen tube grows down to the ovary.
d) Style connects the stigma to the ovary.
e) Ovary contains the ovules (contain single egg nuclei).
f) Filaments support the anthers that contain the pollen. Together they are called the stamen.
This is a diagram of a broad bean
a) Testa protects the plant embryo and the cotyledon food stores
b) Radicle is the embryonic root
c) Plumule is the embryonic stem
d) Cotyledons contain food store for the seed
e) Micropyle is a hole in the testa ( from pollen tube fertilisation) through which water can enter the seed prior to germination
f) Scar is where the ovule was attached to the carpel wall.
Seeds require a combination of
oxygen for aerobic respiration
water to metabolically activate the cells
temperature for optimal function of enzymes
for their successful germination. Each seed has its own particular combination of the above three factors.
It maybe that in a particular species these processes need to be proceeded by other more specialised conditions such as:
fire
freezing
passing through digestive system of a seed dispersing animal
washing to remove inhibitors (beans)
erosion of the seed coat (Poppy)
The particular conditions required by a seed allows it to match germination to favourable conditions
The metabolic events of seed germination:
a) Water absorbed and the activation of cotyledon cells
b) Synthesis of gibberellin which is a plant growth substance. (Hormone is some text longer a term used to describe such compounds).
c) The gibberellin brings about the synthesis of the carbohydrase enzyme amylase
d) Starch is hydrolysed to maltose before being absorbed by the embryonic plant
e) The maltose can be further hydrolysed to glucose for respiration on polymerised to cellulose for cell wall formation.
Flowering Cues:
Plant have to coordinate the production of flowers to coincide with the best reproductive opportunities. There are many environmental cues that affect flowering however the photoperiod is the most reliable indicator on 'time' of year.
The photoperiod the period of day light in relation to dark (night). In northerly and southern regions this photoperiod is a reliable indication of the time of year and therefore one of the most reliable indicators of the seasonal changes.
Short and Long day Plants:
Short day plants (SDP) typically flower in the spring or autumn when the length of day is short.
Long day plants (LDP) typically flower during the summer months of longer photoperiod.
Critical Night Length:
Experiments have shown that the important factor determining flowering is the length of night rather than the length of day. Therefore it may help students to remember
SDP have a critical long night. That the length of night has to exceed a particular length before there will be flowering.
LDP have a critical short night. That the length of night must be shorter than a critical length before there will be flowering.
e.g. Cocklebur is an SDP with a critical night length of 9 hours, when flowering will occur. A shorter night does not produce flowering.
(It is worth noting that this is not the only combination of environmental triggers that bring about flowering. )
Phytochrome System:
The receptor of photoperiod is located within the leaf.
The cellular location of the receptor is unclear.
The chemical nature of the receptor is a the molecule PHYTOCHROME.
Phytochrome can be converted from one form to another by different types of light.
Flowering in SDP:
Short day plants flower when the night period is long.
In day light or red light, Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr). The conversion actually only requires a brief exposure to white or red light.
In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion.
Under short day conditions (long night) at the end of the night period the concentration of Pfr is low.
In SDP, low Pfr concentration is the trigger for flowering.
Flowering in LDP:
Long day plants flower when the night period is short.
In day light (white or red) the Pr is converted to Pfr.
During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration of Pfr.
In LDP, high Pfr concentration is the trigger to flowering.
The mechanism of flowering:
The search for the substance that causes the development of meristematic tissue into flowers was focused on the identification of 'florigen', the flowering hormone.
Phytochrome Fr is the biologically active form of phytochrome but it is florigen. Most research indicates that phytochrome cannot move from the leaf cells to the meristem tissue.
In 2005 a substance, mRNA (FL mRNA) was finally isolated that was found to be moving from leaf to flower meristem.
This mRNA provides a link between the phytochrome system (the receptor), its activation of genes in the leaf (mRNA synthesis) and the differentiation of the meristem into the flower structure.
9.3.1 Structure of an animal-pollinated dicotyledonous plant.
9.3.2 Pollination & Fertilisation.
9.3.3 Dicotyledonous seed structure and function.
9.3.4 Conditions for the germination of a typical seed.
9.3.5 Metabolic processes during germination of a starchy seed.
9.3.6 Phytochrome and the control of flowering.