الأحياء (بالإنجليزية) الصف الثالث الثانوي، الأسبوع 2: How plants move: touch, tendrils, pulling roots and streaming cytoplasm

Cartilages, joints, ligaments and movement

الأسبوع 2 · الفصل الأول (٢): الغضاريف والمفاصل والأربطة والأوتار، والحركة في الكائنات الحية وأنواعها، والحركة في النبات (اللمس والنوم والانتحاء والمحاليق والجذور الشادة والحركة الدورانية للسيتوبلازم) · 3 دروس

How plants move: touch, tendrils, pulling roots and streaming cytoplasm

Biology Sec 3 Term 1, Chapter 1: pp. 10–14

GoalDescribe how a mimosa, a pea tendril and the roots of corms and bulbs move, and say what cytoplasmic streaming shows.

New words

tropism · growth of a plant part towards or away from a stimulus such as light, humidity or gravitytendril · a thin climbing shoot that coils round a supportpulling roots · roots below a corm or bulb that shorten and pull it down in the soilsleeping movement · closing of leaflets in the dark and opening in the light

Explanation

Plants have no muscles, yet they move. The leaflets of a mimosa fold down when touched, a movement by touch. Mimosa and some other legumes partly close their leaflets in the dark and open them again in the light, a sleeping movement. Plant parts also respond to light, humidity and gravity by tropisms. A pea tendril shows a special movement of pulling. The tendril tip reaches a solid support and coils round it. The cause is growth: the side touching the support grows slowly, while the far side grows faster, so the tendril bends round the support. Then the tendril shortens and pulls the stem closer to the support, and it thickens with supporting tissue and becomes stronger. A tendril that finds nothing withers and dies. Corms and bulbs have pulling roots. These roots shorten and pull the corm or bulb down to a suitable depth in the soil, and they help to steady the plant against wind. Finally, inside a living cell the cytoplasm streams. In the leaf cells of Elodea, a water plant, the cytoplasm forms a thin layer lining the wall and flows round in one direction. The green chloroplasts carried along in it show the flow.

Movement	Plant	What happens
By touch	mimosa	leaflets fold down when touched
Sleeping	mimosa and some legumes	leaflets close in the dark, open in the light
Tendril coiling	pea	tendril coils round a support and pulls the stem in
Pulling roots	corms and bulbs	roots shorten and pull the plant down
Cytoplasmic streaming	Elodea leaf cells	cytoplasm flows round the cell in one direction

Worked examples

tip touches a support→touched side grows slowly, far side fast→tendril coils round the support→tendril shortens and thickens→stem is pulled close and held

How a tendril holds on, in order. Each step causes the next.

Sleeping movement follows light and dark. In the day the leaflets are open; at night they partly close.

Pulling roots, with invented numbers. A young corm lies 3 cm below the surface and its pulling roots shorten by 2 cm, so it is pulled down to 3 + 2 = 5 cm. In this way it reaches a safer depth without any muscles.

Class activity

Watch a plant move, with a grown-up. Look at a climbing plant such as a pea or bean in a pot or garden. Place a thin stick near a free tendril tip, never pulling or uprooting anything, and look again each day for three days. Draw what the tendril does and write which side of the tendril must have grown faster. Also compare the leaves of a clover or bean plant at noon and in the evening, and note any difference in position.

Practice

What is the movement called when mimosa leaflets close after being touched? And when leaflets close in the dark?

A tendril touches a wire. Which side of the tendril grows faster, the side touching the wire or the far side?

Why are pulling roots useful to a bulb in a windy place?

What can an observer see in an Elodea leaf cell that shows cytoplasmic streaming?