Download Effect of Light on Growth and Development

Chapter 32
Effect of Light on Growth and Development
1. Vegetative shoot Growth and Development
A Vegetative shoot is one which does not bear any flowers.
Draw: Diagram 30.1 Torrance
Complete:
Vegetative Shoot in
Light
Vegetative Shoot in
Dark
Leaves
Internodes
Stem
Vegetative shoots grown in the dark are said to be etiolated.
Explain: Why is etiolation a survival mechanism?
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2. Flowering in long and short day plants
Photoperiod: Period of illumination.
Photoperiodism: Response to photoperiod.
e.g. Some plants stop producing vegetative (leaf) buds start producing
flower buds in a response to a change in the period of illumination.
Three distinct types of plant exist:
(i)
Long day (short night) plants:
These plants only flower when the number of hours of light to which they
are exposed (photoperiod) is above a certain critical level. (i.e. the
number of hours of darkness is below the critical level) e.g. Spinach.
Complete: diagrams
(ii)
Short day (long night) plants
These plants only flower when the number of hours of light to which they
are exposed (photoperiod) is below a certain critical level. (i.e. the
number of hours of darkness is above the critical level) e.g. Strawberry.
Complete: diagrams
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The length of this critical photoperiod varies amongst different species of
long day short day plants.
Draw: table 30.1
(iii)
Day neutral plants:
These are plants in which flowering is not dependent upon photoperiod.
E.g. celery, tomato, geranium, snapdragon.
Geographical distribution of long and short day plants.
Complete:
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3. The timing of breeding in birds and mammals
Many birds and mammals are seasonal breeders. Their gonads (testes
and ovaries) become active only at certain times of the year.
These changes are triggered by a change in the photoperiod.
Birds: Birds are long day breeders stimulated by an increase in the
photoperiod. i.e. sexually active in the spring and summer.
Why is this a survival mechanism?
Why is it essential that the mass of the gonads be kept to a minimum for
the rest of the year?
Rabbits: They are long day breeders.
Sheep and deer: Complete
Why is being a short day breeder a survival mechanism for sheep and
deer?
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