2019-04-20_New_Scientist

Last words past and present at newscientist.com/lastword
THE LAST WORD

Making new oranges

I have just spent a few hours cutting

up oranges and lemons to make

marmalade, and was startled to see

that none of the oranges had seeds.

How do the trees they come from

reproduce?

Q On the premise that seeds

detract from the appeal of fruit

to consumers, plant breeders

have developed seedless varieties

of a number of fruits, including

oranges and other citrus fruits,

grapes and watermelons. These

varieties can’t reproduce from

seeds, of course, but this isn’t

a problem.

Most commercial fruit trees,

whether seedless or not, are

propagated by grafting. A cutting

is taken from a parent plant and

grafted onto root stock from

which it then develops into a new

tree that (from the graft outwards)

is genetically identical to the

parent. In this way, large numbers

of genetically identical trees with

the desired properties of flavour

and yield can be produced.

Jonathan Wallace

Newcastle upon Tyne, UK

Q A fruit usually forms after

a flower’s ovules have been

pollinated. But in some cases,

fruit can form without

pollination. This phenomenon,

known as parthenocarpy, can

occur as a natural mutation in

some plants. Seedless oranges

come from parthenocarpic plants.

Pollination failures happen in

different ways. For example, navel

orange plants have a mutation

that stops their anthers from

producing pollen. Other varieties,

like Jaffa oranges, have a mutation

that leads to self-incompatibility

between the plant’s sperm and

egg cells.

Because these plants can’t

reproduce through pollination,

they would die out very quickly if

left alone in nature. However, they

are cultivated by farmers who take

cuttings from an original, seedless

orange tree and graft them onto

another, normal orange tree. Each

graft is a clone of the original tree,

which preserves the seedless trait

in the oranges.

So seedless orange plants are

reproducing asexually through

grafting by humans.

Interestingly, all navel oranges

are believed to have come

from a single tree in Brazil that

was found in the early 1800s.

Similarly, according to legend,

Jaffa oranges are based on a

mutation from a single branch

of one tree that was discovered

around 1844 in Palestine. They

are named after their main

export point, the port of Jaffa.

Tiffany and Damian Pang

Discovery Bay, Hong Kong, China

Q Navel oranges are so called

because the end opposite the

stalk looks like a belly button.

The fruit, first discovered in

Brazil, was juicy, sweet, easily

peeled and, having no seeds,

a potential money-spinner.

Cuttings were taken and grafted

onto citrus root stock. These grew

into orange trees that were

genetically identical to the

original, giving consistency of

fruit production. This cultivar

was named the Washington navel

orange as its development was

financially supported by the US

government.

Another parthenocarpic fruit

is the Cavendish banana, which

supplies over 99 per cent of the

bananas in our supermarkets.

David Muir

Edinburgh, UK

Q The implications of this

question are worrying. It sounds

like the questioner isn’t using

Seville oranges to make their

marmalade. Though these tart,

thick-skinned oranges are more

pip and peel than pulp, they are

generally considered to make

the best marmalade.

Adrian Foulds

Glasgow, UK

Cracked it

Why do I, or any other human, get sore

and cracked heels? I understand it’s

less common in men than women –

presumably this is down to footwear

choices? The cream product I use to

cure it works very well, but its active

ingredient appears to be urea. How

does this react with my skin to cure

the problem? (continued)

Q One cause can be sodium lauryl

sulphate (SLS) which is an ionic

surfactant used in many cleaning

and hygiene products. It is

considered safe to use, but, as

with any detergent, it removes

oils from the skin and can be a

source of irritation.

I suffered from dry heels for

a few years and tried switching

from shower gel to normal bar

soap without SLS. There was

a noticeable difference to my

heels within a week and they

have continued to improve.

Most shampoos and

conditioners also contain SLS and

this could contribute to cracked

heels in those who stand in the

shower while washing their hair.

Sarah Harbour

Cambridge, UK

This week’s

questions

CORE SUBJECT

At school we were shown that

heating a bar magnet caused it

to lose its magnetism. How then,

if the iron core of Earth is at a

temperature high enough to

liquefy it, does it generate a vast

magnetic field?

Tony Lamont

Townsville, Queensland, Australia

TOAST, NOT TOAST

Heating bread in a toaster and

a microwave oven creates very

different results. What do

microwaves do to bread?

Reynold D’Souza

Gurgaon, India

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“ All navel oranges are

believed to have come

from a single tree in Brazil

found in the early 1800s”