[Music] in today's video we're going to take a look at how plants can respond to their environment and in particular we'll look at the role of auxins now just like animals plants have to be able to respond to stimuli in their environment in order to survive they just do it much more slowly than animals do for example they can sense light and grow their shoots towards it to maximize photosynthesis they can sense gravity so that they can grow their roots downwards to fix themselves into the soil and find more water and minerals and some plants can even sense touch so that they can climb around other objects in order to do all of this stuff plants have to rely entirely on hormones they don't have any nervous system like animals do and the hormones that they do have normally act locally rather than flowing around the entire organism like they would in animals one group of these plant hormones are the auxins which control growth are the ends of the shoots and the roots once these organs are produced in the tips they then dissolve in the solution of the cells and diffuse backwards along the shoot or the root in the shoots these auxin stimulate growth causing the shoots to grow but in their roots they do the opposite and inhibit growth the two responses we're gonna cover are phototropism which is a response to light and geotropism which is a response to gravity and you might also hear this be called gravitropism but it means exactly the same thing now the key rules that you need to remember are that once the auctions are produced in the tips and they diffuse along they'll always accumulate on a shaded side rather than a sunny side and the lower side rather than the upper side if we start with the shoots we can say that they're positively phototropic meaning that they grow towards the light but that they're negatively geotropic because they grow away from the ground so the opposite direction to gravity to see how this works let's take a look at this shoot because the sun is on its left in this image the right side will be in the shade and so the auxin will accumulate on the right and because auxin stimulate growth in shoots it causes these cells on the shaded side to grow faster than on the sunny side and as you can see it's this difference in growth rate that causes the shoot to curve around towards the light because these cells on the right hand side are growing lots but the cells on the left aren't growing very much if we take this shoot instead we can see gravity will have exactly the same effect the auxin will collect on the lower side here so the cells on this side grow faster than the top side and the shoot will curve upwards now the roots are basically just the opposite negatively phototropic and positively gravitropic so grow away from the light and down towards the ground normally all the plant roots will be underground so don't detect any light but if they do happen to get exposed to light for some reason then the auxins will accumulate on the lower shaded side and remember that in the roots auxins inhibit growth so the upper side will grow faster and cause the root to curve downwards now if we pretend for a second that it curves to the left a bit more so that it's all horizontal we can use this to think about the effect for gravity the auxins are going to accumulate on the lower side and they're going to inhibit growth so this means that the upper side will grow faster and cause the root to curve downwards anyways that's everything for this video so hope you found that useful and we'll see you again soon you