Festive plants

What's the point of being an evergreen plant? And how does holly put off hungry herbivores? Chris Smith spoke to Beverley Glover, the director of Cambridge University’s Botanic Garden. First, Chris asked, what do we actually mean by "evergreen"?

Beverley - Well they do keep their leaves all year round, Chris, so that's what we mean. They might drop a leaf off from time to time when that leaves old and past its lifespan but there'll always be green leaves photosynthesizing on that tree.

Chris - How did the two different types of tree come about, the ones that do drop their leaves and the ones that don't?

Beverley -  Well actually I think there are three different types of trees. Because there are two different types of being evergreen. So if you start at the tropics and think about trees, they're evergreen, they can photosynthesize all year round there's always lots of sun it's warm everything's great. As you move north in the northern hemisphere or south in the southern hemisphere you reach an area like Britain where it's a bit cold and miserable in the winter. You're not going to get much photosynthesis done, there's not much light and your leaves become a big risk because if it's windy they create a big surface area. And so having a deciduous habit where you drop those leaves starts to become evolutionarily sensible it's a good adaptation to survive the winter by dropping the leaves and regrowing them in the spring. But if you carry on North in the northern hemisphere or south of the southern hemisphere you reach a point where the growing season is so short that actually the regrowing new leaves each spring in order to photosynthesize wouldn't make sense. And there's an added double whammy in there which is that the soil temperature is so cold that there's not much fungus around. It's hard for leaves to break down and there's not much mineral nutrient available in the soil. And so for those trees there really isn't time and energy and resource to build new leaves each year and so they have to go back to the evergreen habitat the tropical trees had.

Chris -  So which came first then in evolution? Did the trees that had leaves all year round come first and then they turned into evergreens over evolutionary time or were there evergreen type plants first and they turned into deciduous trees?

Beverley -That's a difficult question because what we now see as trees aren't the first trees that we would have seen in evolutionary time, so ferns and some plants called lycophytes, which are now just little tiny things 30 40 centimetres tall, used to be the trees that we had before we had the current seed plants, and those trees would have been evergreen. They were mostly tropical. And then as the trees started to spread out in the Permian into colder and drier areas then they started to form these deciduous habits and then again the new evergreen forms evolved.

Chris - What's interesting though is that the trees that do drop leaves tend to have big flat leaves whereas the evergreens like Christmas trees have gone for needles. So are needles effectively just modified leaves? And if so why are they like that?

Beverley -  Yeah so that's exactly what needles are and they definitely evolve at a point in evolutionary history where the world gets colder and drier. So the needle form is about protecting yourself against two things: one against a big surface area that the winds can blow through and potentially pull the whole tree over. But secondly against water loss so you're minimizing the surface area of the leaf. The stomata, the pores through which the plant breathes and loses water, are held in channels usually along the needles and so they're sort of below the level of the leaf surface and that reduces the amount of water that's sucked out of them. So it's all an adaptation to retain water and to not get blown over in a cold, dry habitat.

Chris - And do the plants to anything else to tolerate extremes of temperature because a lot of these these evergreens grow in places as you pointed out where it's really really cold for part of the year.

Beverley -  Yes, so lots of plants have different antifreeze type defenses. They'll have cold tolerant proteins that they express when the temperature is particularly cold, that changes the osmotic potential in the vacuole. So that's a big sack of liquid in the middle of a plant cell - plant cells are very different from an animal cell in having a big bag of water sitting in the middle. And if you change what you put in there you can change the temperature at which it will freeze. They also have things like waxes on surfaces which give them an added layer of protection against cold and against drying out. So they've got lots of different things going on at different levels.

Chris -  Other festive plants for a second Beverley, so things like holly and that kind of thing. Tell us a bit about them.

Beverley -  Well holly's a great one because obviously after Christmas lunch you need to get out in the garden get some fresh air and everybody likes a bit of science after lunch!

Chris - In your house perhaps, our lot want to watch the Queen and pass out.

Beverley - Surely everyone wants to see some experiments after lunch? If you go find a nice holly bush in the garden and you can set everybody in the family a challenge count the number of spikes per leaf one foot off the ground, two feet off the ground, three feet off the ground. You should find a statistically significant reduction in spike number per leaf as you go up the plant, which of course is about who's eating those plants, it's an anti herbivore defence designed to protect the plant against rabbits, small deer, grazers at sort of low level!

Chris -  Really?

Beverley - I’m not having you on, go and try it.

Chris - How does the plant know how high up each of the leaves is?

Beverley -  Well that's very clever and it's not just needles, arctic willow for instance puts defensive chemicals in its bark just up to the height an Arctic hair can reach if it stands on its back legs and no higher.

Chris - But obviously the part of the plant that is browsing height when it's growing is quite different than when it's a big mature tree,  so is that an acquired thing as it gets bigger it starts to put more needles lower down and fewer higher up.

Beverley - It's quite a long live tree, holly. So it's got time to sort of acclimate as it ages and there's an element of this which we think has induced in response to where it's been eaten most during its lifetime. But part of it seems to be part of the basic growing design.

Chris - It’s quite literally once bitten twice shy, or twice spiky!

Beverley -  Exactly. Bet you don't know, Chris, that holly is one of those plants that, like animals, comes in male plants and female plants, whereas most plants of course are both. And so if you've got a Holly in your garden that never makes berries it's probably because it's a boy.

Chris -  Is that the only way to tell?

Beverley - You can look at the flowers if you know what you're looking for it's not difficult you should see four stamens on a male and just one stigma on a female. But the best bet is berries.

Chris -  So why has it done that then?

Beverley -  Well it's a bit of an evolutionary dead end we think, about 4 percent of flowering plants go down this route, 96 percent go for the sensible hermaphrodite approach to life. We think they're doing it as an extreme adaptation to prevent self pollination, self fertilization, it's hard for a plant to avoid getting its own pollen on its own stigmas in the flower. This makes absolutely sure you don't, but of course it also means unless there’s another one of you in the same place then you’re not going to reproduce and since they cant move around then reliant on somebody else carrying that pollen for them - that’s quite a risky strategy.

Chris -  It is. And higher up in some trees, especially around here I’ve seen quite a lot of it, is mistletoe. How did that get there and what's special about mistletoe?

Beverley - Its a hemiparasite. So it does do some photosynthesis but it doesn't connect into the soil at all. So it gets all its nitrogen phosphorus from the tree that it’s plumbed into. Too much of it of course and it will weaken a tree over time and it produces those berries at this time of year. And in this country in the UK mistletoe is quite an interesting example of climate change because it didn't used to come very far north, it was always of southern England habitat only, and say here in Cambridgeshire we wouldn't have seen mistletoe 40 years ago. But the berries are spread by Black Caps. Birds that used to not overwinter in the UK, go south to Spain and Northern Africa for the winter. Now the Black Caps are overwintering, they're eating the berries spreading them around and the plant is steadily spreading north across the country with the Black Caps.

Chris - So why did people decide they needed to kiss under it.

Beverley - Well that's not clear. I mean it's some pretty nasty toxins that'll stop your heart quite quickly so maybe there’s a sinister side effect.

Chris - The kiss of death!

Beverley - That kind of thing. Somebody you don’t really fancy but feel you have to kiss.