Support and Transport in Plants, Grade 10: Wilting Is Not Plasmolysis, and the Difference Is Whether the Plant Comes Back

"Wilting is excessive water loss from the soft parts of plants. This results in plasmolysis of the cells and may cause death of the plant."

Wilting is not plasmolysis. They are two different things and the difference is the one a learner can actually use.

In wilting the cell loses turgor but the membrane stays against the wall. Water the plant and it stands up again. In plasmolysis the membrane has pulled away from the wall, and it usually does not come back.

A mealie drooping at four in the afternoon is wilted. By seven it is upright. Nothing that is plasmolysed recovers like that.

This page gives all three units, six corrections, the force that actually gets water to the top of a tree, and a free 75-mark worksheet with its memo.

Where this fits in the curriculum

Subject Life Sciences
Grade 10
Term 2
Topic Topic 5, Support and transport systems in plants
Units Unit 1 anatomy of dicotyledonous plants: the root, the stem, secondary thickening. Unit 2 transpiration: factors affecting the rate, wilting and guttation. Unit 3 transport of water and food: uptake, transport, translocation
Formal assessment None. There is an end-of-topic test, 30 marks, which adds up correctly
Marks Our worksheet is 75

Finding 1: wilting is not plasmolysis

Wilting Plasmolysis
What happens to the cell The vacuole loses water and the cell loses turgor. The membrane stays pressed against the cell wall The cell membrane pulls away from the cell wall
What causes it The plant loses water faster than it takes it up The cell is in a solution stronger than its own sap
What you see Leaves and soft stems droop Nothing, without a microscope
Does it recover? Yes. Water it Usually not

The plasmolysis a learner has already met is the onion cell in strong salt water from the cells topic. That is a different event from a drooping plant on a hot afternoon, and treating them as the same thing removes the one piece of information that makes wilting understandable: it is reversible.

Finding 2: three forces listed as equals, and one of them does nearly all the work

"The forces of transpiration, capillarity and root pressure combine to move water from the roots to the rest of the plant."

That is a list. The ranking is the whole point.

Force How far it can actually lift water
Capillarity in a xylem vessel Well under a metre
Root pressure A few metres at most, and absent in many plants during the day
Transpiration pull All the way to the top of a tall tree

And the end-of-topic test knows it. Its own multiple choice asks which is the most important force causing water to rise up the xylem vessels, and offers root pressure, cohesion forces, capillarity and transpiration pull.

The answer is transpiration pull, and the test is the correction to the summary.

The word the summary leaves out

Cohesion. It is also one of the options in that multiple choice, and it never appears in the revision summary at all.

  • Cohesion is the attraction of water molecules to each other. It holds the water in a xylem vessel together as one unbroken column, from the root hair to the leaf
  • Adhesion is the attraction of water molecules to the vessel walls, which helps hold the column up
  • So when a molecule evaporates off the top, the whole column moves up behind it

That is cohesion-tension, and it is the answer to "how does water get to the top of a tree". Capillarity is a related effect and a much weaker one, and putting it in the list where cohesion should be is what makes the mechanism impossible to explain.

Finding 3: the xerophyte question with three right answers

From the end-of-topic test, for two marks:

"To reduce water loss, a xerophyte has: A Sunken stomata. B A thick waxy cuticle. C Leaves with a large surface area. D Epidermal hairs."

Option Is it a xerophyte adaptation?
A Sunken stomata Yes. The pit traps humid air so the gradient out of the leaf is smaller
B A thick waxy cuticle Yes. It cuts the water lost straight through the leaf surface
C Leaves with a large surface area No. That increases water loss
D Epidermal hairs Yes. They trap a still, humid layer at the surface

Three of the four options are correct. If you are setting this test, accept A, B or D, and then ask the class to explain how each of the three works and why C would make things worse. That is a better question than the printed one.

Finding 4: a potometer measures uptake, not transpiration

"A potometer is a scientific instrument that measures the rate of transpiration under different environmental conditions."

A potometer measures the rate at which a cut shoot takes water up. That is used as an estimate of transpiration, and it is a good one, because most of what a plant takes up it loses.

It is not the same thing. A little of the water taken up is used in photosynthesis and kept in new tissue, so uptake always slightly exceeds loss. One sentence, and it is exactly the kind of distinction a Grade 11 question asks for.

Finding 5: what to expect when you actually run the potometer

The printed results, as average seconds for the bubble to travel 5 cm:

Condition Time What it means
Laboratory, the control 155 s Baseline
Bright light 125 s Fastest. Stomata open wider
Wind from a fan 130 s Fast. Humid air blown off the leaf surface
Plastic bag 160 s Slowest, but only just

A shorter time means a faster rate, which the questions ask about correctly.

But look at the plastic bag: 160 against a control of 155. That is a three per cent difference for the factor that should matter most. A bag over a leafy shoot saturates the air around the leaves and should very nearly stop transpiration.

If your class gets a result like the printed one, something is leaking. Expect the bag to slow the bubble a great deal. It is the most dramatic of the four conditions and the printed table makes it the least.

And the four conditions are not clean comparisons

Bright light also warms the leaf. A fan also cools it. Neither isolates a single factor, and nothing is measured except the time: no thermometer, no humidity reading, no light measurement.

It is still a good practical. Just say out loud that these are conditions rather than variables, which is a point the curriculum cares about and tests elsewhere.

Finding 6: one fibre, two answers

"The xylem in plants forms wood and plant fibres, which are used for many different purposes."

Wood is secondary xylem, and that half is right.

The fibres used for weaving and rope are not. The ilala palm fibre in the basketry case study on that same page, and sisal, and flax, and the hemp in the tissues topic, are all sclerenchyma fibres.

And the tissues topic says so correctly, in its own case study: hemp is useful because it is packed with sclerenchyma, which is long, lignified and dead at maturity.

Same book, two topics, two answers. Wood is xylem. Fibre is sclerenchyma.

One more, while we are here: "schlerenchyma" is misspelt in this revision summary and in the tissues topic. It is sclerenchyma, and it is examined by name in both.

What the material gets right, and some of it is very good

  • The Vaseline leaf investigation in the end-of-topic test is properly designed. Four leaves, one with Vaseline on both surfaces, one on each surface, and one with neither as a control, plus a question asking which is the control and why a control is needed at all. That is better than the equivalent question in the ecology topic, where there is no control and the test asks for one anyway
  • All three pathways across the root cortex are given: through the cytoplasm, through the vacuoles, and along the cell walls. Most treatments give one
  • Guttation is handled properly, with hydathodes, root pressure, wet soil and night-time all in place
  • The ring-barking activity is excellent, and its last question, about bark stripped for muti and the local extinctions that follow, is the kind of question a school book should ask and usually does not
  • The Zulu basketry case study is the best writing in the topic: the ukhamba, the ilala palm fibre, the imbenge woven from telephone wire, and a final question about how little of an overseas sale price reaches the woman who spent four months weaving it

And the basket that leaks on purpose is a lovely piece of physics. The ukhamba is woven tightly but not perfectly, so a little liquid seeps out, evaporates, and cools what is left inside. That is latent heat of vaporisation doing the same job as a leaf.

What this topic costs

Two slides do most of the work, and the cheaper one is the better one.

Item Price On hand What it shows
Root, dicot, t.s. R20,00 1 910 The whole of Unit 1.1 in one view: epidermis, cortex, endodermis, pericycle, xylem and phloem
Stem, dicot, t.s. R29,00 14 The ring of vascular bundles, the pith, the sclerenchyma cap
Leaf epidermis, whole mount R52,00 27 Stomata and guard cells, for the transpiration unit
Monocular student microscope R1 990,00 13 40x, 100x and 400x, which is all this topic needs

The root t.s. at R20,00 is the best value item in the Grade 10 Life Sciences catalogue. It is the cheapest slide we carry, there are nearly two thousand of them, and one view of it covers an entire unit.

The stem t.s. is down to 14, which is not a class set. Ask before planning around it.

What we do not stock

  • A potometer. We do not carry one, and we do not carry capillary tubing either
  • Vaseline. A pharmacy
  • Celery, food colouring, string and a fan. A supermarket and a classroom

The free version of the potometer practical is the Vaseline leaf investigation that the end-of-topic test already describes. Four leaves, a tub of Vaseline, a piece of string and a kitchen scale. It isolates upper against lower surface, it has a real control, and it answers the question the potometer cannot: which side of the leaf loses the water. If you only have time for one transpiration practical, do that one.

If you want the celery and dye demonstration, low form borosilicate beakers start at R20,00 for 25 ml and we hold over 1 300 across the sizes. A jam jar does the same job.

What you should be able to do by the end

  • Name the tissues of a root from the outside in, and say what each does
  • Explain the Casparian strip, and why a plant needs one
  • Describe secondary thickening and explain how annual rings form
  • Define transpiration and give four factors that change its rate, with the effect of each
  • Tell wilting from plasmolysis, and say which one recovers
  • Define guttation and name the hydathodes
  • Give the three pathways water takes across the root cortex
  • Explain how water gets to the top of a tree, with cohesion and transpiration pull
  • Say what a potometer actually measures
  • Explain ring-barking, and why the tree takes weeks to die

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

Related practicals