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Ask a Scientist

Thanks to the Otago Daily Times for running the Ask a Scientist column.

Nectar dependent on plant health

Murray Grant, of Wellington asked:

What is the replenishment time for nectar in plants like flax and pohutukawa that Tui feed on? i.e. how soon can a bird revisit the same flower?

Janice answered:

Your question is a bit like asking “how long is a piece of string”.

Flowers produce nectar to attract animals such as insects and birds, which then transfer pollen to other flowers to achieve cross-fertilisation. Although nectar is costly to produce, the benefit of the plant is that pollen is likely to be transferred more directly to another flower.

In general, nectar-producing flowers will replenish their offerings over the lifetime of the flower. However, the rate at which replenishment occurs depends on plant health, microclimate and flower age. Large, healthy plants will not only produce more flowers, but each flower is more likely to have more nectar and replace it more readily. Hot, dry conditions will cause nectar to evaporate or not be replenished as readily.
Some species will respond to pollinator visits by producing greater amounts of nectar, but for many species, nectar production is fairly constant until the flower reaches a certain age. For plants with hermaphroditic flowers, like New Zealand flax, or harakeke, each flower can function both as a male, exporting pollen to other flowers, and as a female, receiving pollen and producing seeds. When harakeke flowers first open they are functionally male, releasing pollen which is initially orange but which fades to yellow after six to 12 hours. Nectar production is the greatest during the first day and is apparently replenished to some extent even if the flower has been visited. After 18-24 hours, the flower changes to the female phase and is ready to receive pollen from another plant.
Nectar is still produced during this phase, but not in the same quantities as when the flower was releasing pollen.

This difference in nectar production between male and female phases is an efficient use of resources, as the amount of pollen exported to other flowers (success as a male parent) increases with the number of pollinator visits, hence it is worthwhile producing more nectar to attract more visitors.

However, success as a female parent only requires one visit from a pollinator carrying enough pollen to fertilise all of the flower’s ovules. So there is little benefit in replenishing nectar.

Flower expensive and risky process

Room 2, Omaka School, asked:

In spring our snowball tree develops flower balls which are green. After about a week these become white flowers and then the tips of some of the flowers turn pale pink. Why do thee colour changes occur?

Janice answered:

Colour pigments in plants have a variety of functions and what you have do carefully observed most likely related to three different ways in which pigments help plants.
Flowering is and expensive and sometimes risky process for a plant, as a lot of resources are diverted from growth into structures that are vulnerable to attach and may not bear fruit. The flowers of most plants are surrounded and protected by a structure called a calyx, when they are in bud. The calyx segments often look like petals but they are tougher and, in most plants, they are green.
The green colour is due to chlorophyll, which is the main light-harvesting pigment that plants use to convert sunlight into energy. So before the flower has opened, the plant is using chlorophyll to make energy to help pay for the flowers.

As you observed, when the flower has fully opened the white petals become conspicuous and the green calyx is hidden. White flowers are attractive to a range of pollinating insects such as flies and bees, so here colour is helping the flower to be pollinated and set seeds.

I imagine you snowball tree is swarming with insects on a calm sunny day.

Damage from sunburn, and attack from flower eating insects, can trigger the production of another important plant pigment: anthocyanin. Anthocyanins are responsible for most of the red, people and blue colours we see in plants and have an important role as antioxidants, mopping up the effects of damage to plant tissues. So the changing colours you have observed on your tree represent just some of the ways that pigments help plants harvest energy, attract pollinators and protect themselves from damage.

A feature that ads rapid dispersal

Amy Sellwood of Belclutha Primary School asked:

Are three sycamore seeds stuck together unusual?

Janice answered:

No, it is unusual to see three sycamore seeds stuck together.

Sycamore is in the maple family and maple fruits are usually what are called a double samara, which is a pair of winged, joined seeds. Winged seeds as a dispersal mechanism are often a feature of rapidly growing trees that specialise in colonising gaps or disturbed edges of forests. When released from a height, the seeds are able to catch air currents and potentially reach new gaps of disturbed areas.

Dispersing seeds like this is relatively cheap as the plant hasn’t had to invest in sugary pulp to attract animals to eat the seeds.

When seeds are cheap to produce, the plant can afford to make more of them, which is an advantage when gaps in the forest might be far and few between.

Sycamore is an invasive tree, particularly in urban bush remnants, largely due to its rapid growth and highly effective seed dispersal. Native trees which use this dispersal mechanism include Dononea (akeake) and Hoheria (lacebark). The ability of the seeds to travel depends on the area of the wing in relating to the weight of the seed.

Paired seeds are likely to be better balanced and so keep the seed in the air for longer. If you still have your triple samara you could conduct your own scientific experiment to see how long it stays in the air and how far it travels comparted to a normal double samara and a single samara (split a double samara in half). Then you could compare distance travelled or time aloft with the wing area to seed weight ratios of each type of samara. Your triple samara may turn out to be a mutant super-flyer (or a lead balloon!). 

White fur reduces water loss

2013 - Mitchell Gunn, of Room 10, Balclutha Primary School, Lanark St, Balclutha 9230, South Otago, asks:

Why has the leaf I found at school got white fur on the back? Is it to keep the moisture in it?

Janice answered:

Many leaves are furry or hairy. Even when a leaf looks smooth, a magnifying glass, or microscope will often reveal tiny hairs, particularly on the back of the leaf. Botanists use a range of scientific terms to describe the different hairs on plants, including glandular (a stalk with a “pin head” containing, for example, a chemical that deter bugs), stellate (branched like a snowflake) and my personal favourite, arachnoid, meaning spidery. The type of hairs on a plant can be important in identification, for example, yellow-flowered daisies you probably have around school, like Dandelion, Catsear, Hawkbit and Hawksbeard, differ in whether their leaf hairs are simple, segmented or stellate. Dense white fur on the underside of a leaf, scientifically called tomentum, is particularly common in the daisy family. The tomentum of Tikumu, large native mountain daisies, is so dense it can be stripped off (see picture) and was used for clothing and insulation by southern Māori. White furry leaves are also seen on plants in cold, high light areas like high mountains or in dry areas, like deserts.

So why are leaves furry? You are right in thinking it has something to do with water. Leaves produce food for the plant via photosynthesis. In order to turn sunlight into carbohydrates, chloroplasts inside the leaf need carbon-dioxide and water. Pores in the leaf allow carbon dioxide in, but they also allow water to escape. When a plant has plenty of water this isn’t a problem, in fact losing water through the pores can stop the leaf from overheating, just like your sweat cools you down. However in cold or dry places where water can be limited, the plant needs to reduce water loss while avoiding overheating. Plants in these places often protect their pores in deep furrows under a layer of hairs, often on the underside of the leaf. This way the leaf can let in carbon dioxide and cool down without losing too much water to evaporation. Plants that are silvery furry all over can even further limit water loss and overheating by reflecting back the high levels of light that are typical of high mountains and deserts.

Totara masting mystery

Judy Simpson, of Karori, asks :

In our garden we have a totara which is nearly 90 years old. Last summer there was a mast flowering with a huge crop of ripe drupes. Most years there are a few ripe drupes, sometimes it seems none. There is another totara tree, I think male, about one hundred metres away from our tree. Some years there are lots of seedlings from the tree in our garden but other years we don’t notice any. In a nutshell my question is; Can the female “flowers” of the totara ripen into fleshy drupes even if no pollen from a male tree has fertilised them? Can they ripen but not be fertile and so they don’t produce seedlings?

Janice answered:

Like most New Zealand conifers, Tōtara "masts", which means it has heavy fruiting years interspersed with years with little or no fruit. This strategy is thought to minimise losses to specialised seed predators (mainly insects) by "predator satiation", i.e. flooding the market with more seeds than can be eaten. For this strategy to work there has to be "off" years in which the seed predators have little or no food so their numbers are reduced. In the case of wind-pollinated, fleshy-fruited plants like Tōtara, it is also thought that masting improves the efficiency of pollination because there are more pollen grains in the air at any one time. Masting may also benefit fruit dispersal by attracting more dispersers from a wider area. The fact that you see some fruit apparently forming, but then failing to germinate, is related to the specific way in which conifers go about seed production. Conifers, along with cycads, ginkgo and a few other species, are known as "gymnosperms" and they differ from flowering plants (angiosperms) in the timing of investment in developing seeds. In flowering plants, very few resources are put into the ovules (these are located in the ovary at the base of the flower and each contains an egg cell) prior to pollination. It is only after pollination that the endosperm in the seed starts to develop and the seed enlarges. In gymnosperms, a large amount of resources is invested in the structures associated with the ovule (there is no ovary) before pollination, for example in the Australian cycad Macrozamia, each of these structures (technically known as mega-gametophytes) can grow to nearly 80% of full seed size even if it hasn’t been pollinated. Sterile, half-developed fruit, is more likely to be seen in years were there is little pollen available. The increase efficiency of the angiosperm system is thought to be a major reason why they dominate in most habitats around the world.

Pussy willow unusual

R Gordon, of St Clare, asked:

Where are the seeds of the pussy willow and how are the dispersed?

Janice answered:

Pussy Willow is the common name of Salix x Reichardtii, the "x" denoting that it originated as a hybrid between two different species - S. caprea and S. cinera.

All members of the willow family have separate male and female trees, i.e. they are dioecious. In spring both sexes produce catkins, which are elongated inflorescences containing many tiny flowers. The structure of a catkin is simple. Flowers are attached individually to a central stalk called the rachis and each flower has a single oblong bract this base, which is often hairy.

Flower petals are reduced to one or two squarish to oblong "glands", which are less than one millimetre long. male flowers usually have two pollen-producing stamens. Female flowers have a single ovary and no stamens. The ovary matures in to a dry capsule which splits into two, releasing vast quantities of minute seeds. Seed fall is not very conspicuous because as well as the seeds being tiny, they are released well after the leaves have expanded. Due to the small size of the seeds they are easily carried aloft by even light wind.

Pussy willow is unusual in that the common name is only applies to the mail plants because only the male catkins are the real "pussies", being compact and softly hairy. Female pussy willow trees do exist but as the catkins are more elongated and less hairy they don’t look like real pussy willow.

Leaf fall of some trees continuous

Keegan Russell, of Hastings Intermediate, asked:

Why are most of our native trees evergreen?

Janice answered:

Autumn highlights one of the most profound differences between native New Zealand trees and trees from temperate northern hemisphere regions. While visitors and locals alike marvel at the glorious colours of poplars, maples and many other northern hemisphere deciduous trees, our native trees mostly don’t look that different.

Leaf fall in winter-deciduous trees is generally triggered by a combination of colder temperatures and shorter day lengths. The colour change we love occurs as a result of pigments and other compounds being dismantled or transported back into the stem as the plant actively cuts off its leaves.

In regions where summers are predictably warm and suitable for growth and photosynthesis, but winters are predictably unsuitable for growth, leaf fall is thought to be economical as it reduces damage from frost, wind and snow and also conserves water, as leaves lose water through their pores (stomata). Evergreen trees in cold climates, such as pines, have small leaves that are well protected with a thick waxy coating, and branching patterns that help shed snow.

Our oceanic climate means New Zealand summers are not always reliably warm and New Zealand winters are not always consistently too cold for growth. This is thought to be the main reason why so few native trees have evolved to drop all their leaves in autumn – they might miss a chance to grow!

However, New Zealand trees don’t keep their leaves forever; some species replace them more or less continuously, but other species such as the southern beeches (Nothofagus) have distinct spring or autumn periods of leaf fall.

If you want to know when your favourite tree changes its leaves, mark some leaves with small dot of paint on the underside and watch them. You might be surprised to find our trees do drop leaves in autumn, just not all at the same time.

Column publication information

Lord, J. 2016. Leaf fall of some trees continuous. Otago Daily Times, July 8th “Ask a Scientist”.
Lord, J. 2015. Seed dispersal agent unclear. Otago Daily Times, July 31st, page 12. “Ask a Scientist”.
Lord, J. 2013. White fur reduces water loss. Otago Daily Times, July 12th, page 12. “Ask a Scientist”.
Lord, J. 2013. Nectar dependent on plant health. Otago Daily Times, Jan 18th, page 14. “Ask a Scientist”.
Lord, J. 2009. Flowering an expensive and risky process. Otago Daily Times, Oct 16th, “Ask a Scientist”.
Lord, J. 2009. A feature that aids rapid dispersal. Otago Daily Times, October 23rd, “Ask a Scientist”.
Lord, J. 2006. Pussy Willow unusual. Otago Daily Times, Oct 4th, “Ask a Scientist”.
Lord, J. 2003. Colour key to pollination. Otago Daily Times, Sept 19th, pg. 20, “Ask a Scientist”.