1 00:00:05.130 --> 00:00:08.770 Earlier in today's lecture, I said that proprioception is 2 00:00:08.770 --> 00:00:12.770 not only knowing up and down, but it's also balance. 3 00:00:12.770 --> 00:00:15.380 It's knowing where the body parts are when you're moving. 4 00:00:16.580 --> 00:00:18.960 When a ballet dancer leaps across the stage, and 5 00:00:18.960 --> 00:00:22.900 lands in an Arabesque, he's not only perfectly balanced. 6 00:00:22.900 --> 00:00:27.090 He's acutely aware of the position of every part of his body. 7 00:00:27.090 --> 00:00:29.400 He knows how far his leg is stretched behind him. 8 00:00:29.400 --> 00:00:31.130 I'm not going to show you that here. 9 00:00:31.130 --> 00:00:34.440 How high his hand is, and what is the angle of his torso. 10 00:00:37.200 --> 00:00:41.260 It's no surprise, though, that we regard plants as stationary beings. 11 00:00:41.260 --> 00:00:44.700 They're sessile organisms, eternally rooted. 12 00:00:44.700 --> 00:00:46.920 And incapable of locomotion. 13 00:00:48.370 --> 00:00:51.013 But as we saw in some of the time lapse movies. 14 00:00:51.013 --> 00:00:54.320 When we look at them patiently over a long period of time, 15 00:00:54.320 --> 00:00:56.850 their stationary stature gives way to 16 00:00:56.850 --> 00:01:00.610 an intricately choreographed festival of movement. 17 00:01:00.610 --> 00:01:02.242 Much like Baryshnikov springing to life 18 00:01:02.242 --> 00:01:04.410 in the first scene of a ballet. 19 00:01:04.410 --> 00:01:09.680 Leaves curl and unfold, stems move and twist, and flowers open and close. 20 00:01:11.260 --> 00:01:13.890 Just take a look at this movie of sunflower seedlings. 21 00:01:16.450 --> 00:01:22.990 We see these seedlings turning in circles. This is called circumnutation. 22 00:01:22.990 --> 00:01:26.038 It's a term that was coined by, again, none other than Darwin. 23 00:01:26.038 --> 00:01:29.010 A large part of the power of movement 24 00:01:29.010 --> 00:01:32.900 in plants, Darwin talks about plants moving in circles. 25 00:01:32.900 --> 00:01:37.100 What he would do, would be stay up for hours and with a glass 26 00:01:37.100 --> 00:01:41.569 plate above a plant, mark on the plate the position of the tip of 27 00:01:41.569 --> 00:01:42.644 the chute. 28 00:01:42.644 --> 00:01:47.010 And he noticed that plants would move in spirals and in circles. 29 00:01:47.010 --> 00:01:50.663 Here's actually a picture of one of his traces from his book. 30 00:01:51.720 --> 00:01:55.412 He did this with numerous plants and what he saw that each plant 31 00:01:55.412 --> 00:01:58.678 had its own characteristic shape of circumnutation, 32 00:01:58.678 --> 00:02:01.740 its own period, and its own speed. 33 00:02:01.740 --> 00:02:04.230 But every plant tested circumnutated. 34 00:02:06.960 --> 00:02:11.280 This led to two different hypotheses about why plants circumnutate. 35 00:02:12.470 --> 00:02:15.760 Darwin said in the 1880s that circumnutation is 36 00:02:15.760 --> 00:02:19.160 hard wired into the behavior of all plants. 37 00:02:21.110 --> 00:02:22.770 Almost 100 years later though, there was 38 00:02:22.770 --> 00:02:26.210 an alternative hypothesis by two Swedish scientists, 39 00:02:26.210 --> 00:02:28.850 Israelson and Johnsson, and they claimed that 40 00:02:28.850 --> 00:02:32.430 circumnutation is just a result of gravitropism. 41 00:02:33.720 --> 00:02:37.790 In other words, the spiral movement of the plant, is 42 00:02:37.790 --> 00:02:41.920 a result of plant growth overshooting where it should be. 43 00:02:41.920 --> 00:02:46.720 You might remember the first movie that we showed, showing that early plant 44 00:02:46.720 --> 00:02:51.820 gravitropism, the plant went like this, and then like this, and then like this. 45 00:02:53.950 --> 00:02:57.620 What Johnsson said was that as a plant somehow 46 00:02:57.620 --> 00:03:01.010 bent is falling a bit to the left, for example. 47 00:03:01.010 --> 00:03:04.190 Gravitropism would then make it bend upwards. 48 00:03:04.190 --> 00:03:07.576 But it then overshoots and then goes a bit to the right. 49 00:03:07.576 --> 00:03:11.750 Now the aminoplasts would fall again to the bottom, it would be responding to 50 00:03:11.750 --> 00:03:16.690 negative gravitropism, try to go up, overshoot and go a bit to the left. 51 00:03:16.690 --> 00:03:19.240 And back and forth, where you would get a waving 52 00:03:19.240 --> 00:03:23.450 movement which would be going in spirals. So what they were saying is that 53 00:03:23.450 --> 00:03:28.320 circumnutation is not hard-wired, but is only a result of gravitropism. 54 00:03:28.320 --> 00:03:35.100 So how can these two hypotheses be tested? How can we differentiate between them? 55 00:03:38.830 --> 00:03:42.295 We could differentiate between them using two tools. 56 00:03:42.295 --> 00:03:44.166 The first would be genetics. 57 00:03:44.166 --> 00:03:47.962 Can we find mutants that are defective in circumnutation 58 00:03:47.962 --> 00:03:52.571 or test mutants that are defective in gravitropism for circumnutation? 59 00:03:52.571 --> 00:03:56.567 What it actually ends up with is that if you take in an arabidopsis 60 00:03:56.567 --> 00:03:59.897 mutant that has no aminoplasts, arabidopsis mutant 61 00:03:59.897 --> 00:04:04.710 that has defective gravitropism, also have defective circumnutation. 62 00:04:04.710 --> 00:04:07.940 We see that actually sometimes in ornamental plants. 63 00:04:07.940 --> 00:04:10.492 For example, there's a morning glory that falls, 64 00:04:10.492 --> 00:04:13.560 or the chutes falls, it's a hanging plant. 65 00:04:13.560 --> 00:04:15.470 The reason is that this morning glory falls 66 00:04:15.470 --> 00:04:18.409 is that its chutes are defective in gravitropism. 67 00:04:19.460 --> 00:04:23.160 And these chutes of morning glory are also defective in circumnutation. 68 00:04:24.200 --> 00:04:30.090 So the genetics would support Johnsson's hypothesis that 69 00:04:30.090 --> 00:04:34.010 all of the turning is due solely to gravitropism. 70 00:04:34.010 --> 00:04:34.510 The 71 00:04:36.170 --> 00:04:39.369 other way of testing this hypothesis would be in space. 72 00:04:40.430 --> 00:04:43.330 And it was tested in 2007, interestingly in 73 00:04:43.330 --> 00:04:47.190 an experiment that was designed by the same Johnsson. 74 00:04:47.190 --> 00:04:51.630 His hypotheses as a young scientist in 1968, was finally 75 00:04:51.630 --> 00:04:55.210 tested in 2007 when he was a very senior scientist. 76 00:04:56.960 --> 00:04:58.890 So aboard the space sta, the International 77 00:04:58.890 --> 00:05:01.810 Space Station, they took seeds that were germinated 78 00:05:01.810 --> 00:05:03.080 in space. 79 00:05:03.080 --> 00:05:05.450 In other words that these seedlings had never been exposed to 80 00:05:05.450 --> 00:05:10.410 gravity, and then tested using a camera, whether there was any circumnutation. 81 00:05:10.410 --> 00:05:14.330 And what they found was that there were 82 00:05:14.330 --> 00:05:18.000 very, very minute movements of these seedlings in space. 83 00:05:19.180 --> 00:05:22.250 Almost undetectable, but there were small 84 00:05:22.250 --> 00:05:24.729 circular movements in the absence of gravity. 85 00:05:26.470 --> 00:05:26.830 Then when the 86 00:05:26.830 --> 00:05:30.360 scientists aboard the space station took the same seedlings and 87 00:05:30.360 --> 00:05:34.970 put them in the centrifuge, then the movements got much larger. 88 00:05:36.460 --> 00:05:39.115 So on the one hand, Darwin was correct. 89 00:05:39.115 --> 00:05:43.630 Circumnutation appears to be an inherent feature of all plants. 90 00:05:44.990 --> 00:05:48.360 Where was Johnsson correct? 91 00:05:48.360 --> 00:05:52.790 He was correct in that gravitropism then mediates the period, or 92 00:05:52.790 --> 00:05:56.040 increases how fast and how far it's going. 93 00:05:58.080 --> 00:06:01.420 So a plant can be pulled in many directions, as we've seen. 94 00:06:01.420 --> 00:06:05.940 Sun light coming from the side can pull it to bend towards the sun. 95 00:06:05.940 --> 00:06:10.620 Whereas, gravitropism at the same time, would be causing it to grow up. 96 00:06:10.620 --> 00:06:15.220 We saw with the cuscuta plant that a smell might be causing it to go to the side. 97 00:06:15.220 --> 00:06:17.840 But at the same time, its aminoplasts would be 98 00:06:17.840 --> 00:06:19.879 falling to the bottom, causing it to grow up. 99 00:06:21.080 --> 00:06:23.490 These often conflicting signals 100 00:06:23.490 --> 00:06:26.050 enable to a plant to situate itself 101 00:06:26.050 --> 00:06:29.200 in a position that's optimal for its environment. 102 00:06:30.410 --> 00:06:33.270 The tendrils of a vine, the part of a vine that circles 103 00:06:33.270 --> 00:06:37.800 a fence, search for the fence may be attracted to the shade 104 00:06:37.800 --> 00:06:41.520 of the neighboring fence, while gravity will enable it to wrap a 105 00:06:41.520 --> 00:06:45.470 twirly, will, will enable it to circle rapidly and twirl around the fence. 106 00:06:46.520 --> 00:06:48.770 A plant on a windowsill will be pulled 107 00:06:48.770 --> 00:06:52.210 by light to grow to one side toward the sunny part, while the 108 00:06:52.210 --> 00:06:55.110 force of gravity will influence it to grow up at the same time. 109 00:06:56.340 --> 00:06:59.270 Just like in Newtonian physics, a position of any 110 00:06:59.270 --> 00:07:02.520 part of the plant can be described at one time 111 00:07:02.520 --> 00:07:04.830 as a sum of all of the vectors that act 112 00:07:04.830 --> 00:07:08.070 upon the plant, to tell it both where it is. 113 00:07:08.070 --> 00:07:09.320 And what direction to grow. 114 00:07:10.860 --> 00:07:14.690 And a plant without proprioception would be unable 115 00:07:14.690 --> 00:07:18.770 to find its optimal place in its environment for continued growth. 116 00:07:20.660 --> 00:07:22.990 Thanks for being me with this with this lecture. 117 00:07:22.990 --> 00:07:24.140 We'll meet again next week.