Celery and xylem

Dave Ansell shows Adam Murphy how plants get water from their roots to their leaves, using some rather sad celery...

Dave - For this demo, what you want is, ideally, some nice fresh carnations, or something with nice big white petals, which is just coming into bloom and is really enthusiastic. However, it's a bit hard to get hold of at the moment. So what I've got is some slightly ill looking daisies and some very ill looking celery.

Adam - And I didn't want to go off and steal my neighbour's flowers. So all I have is some celery

Dave - And you also want a little bit of water in a cup, or a glass, and something to colour that water with. So either some food colouring or maybe an old felt pen, which isn't really dead, washable felt pens, which can pull the sponge in the back out of, and get the ink out of it. If you are opening up a felt pen, be very careful, especially the ones designed for younger children, are incredibly hard to open. So get an adult to do that.

Adam - Okay, so I've got a jar and I'm going to fill that up with water.

Dave - I'll do the same. And then you want to colour it with food colouring or some ink. So it's fairly dark, not completely opaque. You don't want it with so much ink in there that it's kind of syrupy, but you want it so it's definitely very strongly coloured.

Adam - I'm going to open this bottle of red food colouring that I have. It's a very dark red food colouring and I'm going to just open it up, and then pour in. And now I have a jar of dark red liquid, which looks very suspect.

戴夫,我刚洗了的海绵of a child's felt pen, and I'm just squeezing the ink out of it into the water. So I'm getting some really deep purple water. I'm not 100% sure which of these is going to work best, but we can both try one and see which is the best way of colouring water. Okay, so mix in that ink or food colouring. And I now have very purple fingers. So now we want to treat your flowers or the celery as if it was normal cut flowers. So you want to cut them probably diagonally, put them into the water and then leave them for a while.

Adam - Well, I've got my sharp knife. And if you're doing this at home, please remember to have a grownup with you while you do this. And I'm cutting the bottom of my celery. And now I have freshly cut celery and this goes in the jar, Dave? Yes?

Dave - Yeah. So I'm doing the same thing with my slightly ill looking daisies, and my very ill looking celery, put it in the jar as if it was a vase, then you want to put it on a nice bright windowsill, where it's reasonably warm and then we'll come back in a few days and see what happens.

Katie - Which funnily enough, is exactly what they did.

Dave - So it's been three days and I've got various flowers and celery in various different coloured liquids. How have yours done Adam?

Adam - Mine have not gone particularly well. I just have some celery sitting in some red water. It hasn't really done anything. What about your end, Dave? What have you gotten?

Dave - So I've got one, which is basically exactly the same as yours, which is just some celery sitting in some red water. And I've got some daisies sitting in some blue water, which has done virtually nothing. I think both of these were in food colouring. That's what you used wasn't it?

Adam - It was indeed, red food colouring.

Dave - However, the one I used in ink from a felt pen has worked really nicely. The leaves are all nice and purple. It looks like almost a different breed of celery.

Adam - Why has one worked, and one not worked?

Dave - So I think this was a mistake I made, which was that food colouring has changed since I was a child. In the olden days, food colouring was made of all sorts of lovely, very soluble chemicals, made in a chemical factory, which were very strongly coloured. Whereas these days, because those are possibly not ideal for children, they've converted lots of vegetable dyes which have got bigger molecules, and they're probably more likely to stick to organic things, to plants. And so if I slice up my celery, in fact, do you want to try this too Adam? Just take a series of small slices off the bottom of your celery, see if you can see the colour working its way up the celery at all.

Adam - So if I look really closely, there's like one or two red veins working their way, maybe a centimetre or two up to celery. That's about it.

Dave - Yeah, that's what I've found with my celery. If I cut down, after about a centimetre, they stop entirely, on the celery from the food colouring, because what's going on in the plants, is they've got little tiny tubes which go from the roots up the leaves, which are way of solving a problem that plants have, which is how to get all the useful nutrients up from the roots up to the leaves, because they don't have a heart. They have no way of pumping it actively. So what they have, is they have these long thin tubes called xylem, which go from the leaves down to the roots, and whilst the roots do push the water up a little bit, mostly what's happening is that water evaporates from the leaves, and that reduces the pressure in these tubes, and that just basically sucks more water up through these long xylem and water and the nutrients get taken up with it.