• Question: Because in the desert water is scarce, are cacti more fragile due to the fact water is needed to produce glucose, and glucose produces cellulose, which is needed to build new call walls? Is this the reason Cacti have spikes, so as to defend themselves because their cell wall is hard to recreate?

    Asked by to Aimee, Chris, Dave, Greig, Laurence on 16 Jun 2014. This question was also asked by , .
    • Photo: Aimee Hopper

      Aimee Hopper answered on 16 Jun 2014:

      Good question. What you say here is correct – water is needed to make glucose to build the constituent parts of the cacti. I believe this is why cacti are very thick – this provides the stiffness they need not to fall over and break.

      The spikes are there for 2 reasons. 1) stops other creatures eating them, and 2) the spikes are actually the leaves of the cactus. They have adapted to be thin and pointy to reduce water loss from the leaves, but still need to be there to maximise the amount of C02 the plant can absorb.

    • Photo: Dave Jones

      Dave Jones answered on 16 Jun 2014:

      Cacti are pretty tough, and I think that is to do with having very little water. They have evolved to be able to survive in the harsh desert environment where other plants would die. To do that they have to be able to survive with little water, but when they do come into contact with water (because it does rain in the desert just not very often at all) they have to be able to store it and use it until it rains again. In order to do that they have had to become tough and strong, with spikes so that the water can’t escape (through leaves or whatever) and also so that desert animals don’t tear them open to get to the water stored inside.

    • Photo: Laurence Perreault Levasseur

      Laurence Perreault Levasseur answered on 17 Jun 2014:

      Very good question! I think your logic is right, but I think the conclusion is that cacti are more resistant. The little stem they slowly build must be very tough in order to protect them as much as possible form 1) from water evaporation since it would be very bad if they were to loose lots of water that way and 2) time wear and attack from other animals, because if they get ‘injured’ it would be very long and time-consuming to recover (and you’re right, that’s a reason why they grow spines)
      3) They also need to have strong and resistant walls because in their core they have important reserves of sweet water, which, on top of making 1) and 2) more important, is very heavy and so they need strong membranes to support that weight.
      Also, the stem of cacti is very special, because that’s where most of the photosynthesis happen. Some cacti also have leaves, but the spines don’t do any photosynthesis.

Comments

  • Photo: Laurence

    Laurence commented on :

    hey Aimee, I don’t understand how the spines help maximizing the amount of CO2 absorbed… can you explain please 😀
    From wikipedia I just read: ‘Spines provide protection from herbivores and camouflage in some species, and assist in water conservation in several ways. They trap air near the surface of the cactus, creating a moister layer that reduces evaporation and transpiration. They can provide some shade, which lowers the temperature of the surface of the cactus, also reducing water loss.’