now in this particular video we want to talk a little bit about a group of plants referred to as something known as zeroes zeroes are just basically plants that live in extremely dry conditions examples of zero phyes are CTI or cactus that live in desert you see some problems that plants face if they are living in the desert is the fact that the desert has a lack of soil water and it has a very low humidity and because it has very low humidity and also high temperature by the way but more more in particular the low humidity the low humidity factor is the one that is more important because when the humidity is extremely low the plants may lose too much water vapor through transpiration and if it loses too much water it will Wilt and die so how do zeroes or plants that live in dry conditions adap to survive in this area one way that they can actually adapt to survive is by having thick cuticles on the leaf cuticles are just the waxy layer that are usually found on the epidermis of the leaf so I'm going to draw out two cross-sections of the leaf here and as you can see the leaf on the left has a thin layer of cuticle but the leaf on the right has a thicker layer of cuticle so how does the thicker layer of cuticle reduce transp ation rate as you can see here what happens is the amount of light going into the leaf is represented by the arrow so the leaf on the left because the cuticles have a thin layer a lot of light are able to enter but the thicker layer of cuticle will actually reflect more light and reduce the amount of light going into the leaf therefore it actually reduces the Lea's internal temperature and when it reduces the leaf internal temperature less water gets evaporated less kinetic energy of water vapor so therefore less transpiration will happen another thing that the thick cuticles can also do is it reduces evaporation from the leaf surface directly so this is how the thick cuticles May reduce the rate of transpiration for zeroes another way that uh plants can survive in dry conditions is by directly reducing the leaf surface area the larger the surface area of the leaf on the left it will have more stomata but if the leaf is extremely small and needle like or spin like they will just have less stoa as evident when you look at Cactus because when you look at the needles of the cactus those are the leaves technically speaking but the leaves have reduced in surface area over many generation through a process of evolution so because the leaves are so small and have extremely small surface area they have lesser stomata and when they have lesser stomata lesser transpiration will happen because water vapor needs to move through the stomach that's another method how zeroy survive in dry conditions now the third condition is the most important one because they will love asking this in the exam what the plant does is the plant May reduce the steepness of the concentration GR Ian between the inside and outside of the leaf now what do I mean by that what I mean by this is remember let's go back to concentration gradient if the plant has a very steep concentration gradient of water vapor too much transpiration will take place but if the plant has a less steep concentration gradient of water vapor less transpiration takes place now remember plants that live in dry conditions will constantly have a very steep concentration gradient of water vapor because the air outside the leaf constantly has a low humidity so no matter what happens water vapor will constantly diffuse out of the leaf through diffusion so more transpiration will take place because it has a steep concentration gradient so the plant has to reduce the steepness of the concentration gradient but that's impossible how is the plant supposed to make the outside air more humid that's like asking a single plant to make the entire desert humid is that possible how can one plant make the outside more humid right but it's not as impossible as it seems actually the plants cheat and hack the plants sort of cheat their way through this system you see one way the plant can reduce the steepness of the concentration gradient is by having something known as tricomes or Leaf hairs imagine the leaf on the left hand side okay uh the Water Vaper the moment it escapes it the moment the water vapor diffuses out of the leaf it escapes into the surrounding environment and never looks back it just diffuses into the surrounding to occupy the empty space but on the right side the moment the water vapor diffuses out it cannot Escape into the environment immediately because the Leaf hair wraps the water vapor outside the leaf what that does is it creates a slightly humid area directly outside the leaf so look at the concentration gradient between the two areas for the one on the left they have a steep concentration gradient but the one on the right has a less steep concentration gradient right now so what happens to transpiration on the right side it will become it will become lesser that's one way another way is by the leaf to have something known as a sunken sto okay sunken sto just means that the sto is depressed inwards as I'm showing you in this particular diagram how does that help you see when the water Vaper diffuses out of the leaf right here look at the diffusion distance between the inside and outside environment okay for the normal Leaf the diffusion distance of the water vapor is short but for the sunken stomata the leaf with the sunken stomata the diffusion distance is much longer and you know for a fact that when the diffusion distance increases less diffusion takes place that's number one and also when the water vapor goes out of the leaf through transpiration it doesn't just immediately Escape into the air it will still remain in that space uh just outside of the stomach and that creates a moist space right and when it creates a moist space this also decreases the concentration gradient of the water vapor thus less transpiration takes place and the third fantastic thing that leaves may do is by having rolled up leaves look at this normal Leaf over here as you can see the water vapor the water vapor can just immediately Escape into the environment okay and get lost but in z ites some of their leaves will roll up and when their leaves roll up the water is also Escaping The Leaf but look it creates a moist environment within the rolled up space and when it creates a moist environment within the rolled up space that also helps to reduce the steepness of the concentration gradient between the inside and outside of the leaf these are pretty fantastic ways plants can humidify the air outside the leaf without humidifying the entire desert because it's impossible to humidify the entire desert but they able to humidify the space just outside the leaf by having Leaf hair sunken stomata or rolled up leaves by having these adaptations they can reduce the steepness of the concentration gradient thus reducing the transpiration rate