The fifth programme explores the alliances formed between the animal and plant worlds. Attenborough dives into Australia's Great Barrier Reef and contrasts the nocturnal feeding of coral, on microscopic creatures, with its daytime diet of algae. Some acacias are protected by ants, which will defend their refuge from any predator. Besides accommodation, the guards are rewarded with nectar and, from certain species, protein for their larvae as well. Fungi feed on plants but can also provide essential nutriment to saplings (Mycorrhiza). The connection is never broken throughout a tree's life and a quarter of the sugars and starches produced in its leaves is channelled back to its fungal partners. Meanwhile, fungi that feed on dead wood leave a hollow trunk, which also benefits the tree. Orchids enjoy a similar affiliation. Lichens are the product of a relationship between fungi and a photosynthetic associate, usually algae. They are extremely slow-growing, and a graveyard is the perfect location to discover their exact longevity. Mistletoe is a hemiparasite that obtains its moisture from a host tree, while using own leaves to manufacture food. Its seeds are deposited on another by the mistletoe tyrannulet, following digestion of the fruit. The dodder (Cuscuta) is also parasitic, generally favouring nettles, and siphons its nourishment through periodic 'plugs' along its stem. The rafflesia has no stem or leaves and only emerges from its host in order to bloom — and it produces the largest single flower: one metre across.

Sir David Attenborough reveals plants as they have never been seen before - on the move and dangerously devious. About the major problems of life - growing, finding food, reproduction - and the varied ways plants have evolved to solve it. Filmed from the plant's point of view, using computer animations, fibre-optics and unique time-lapse photography. The first episode looks at how plants are able to move". The bramble is an aggressive example: it advances forcefully from side to side and, once settled on its course, there is little that can stand in its way. An altogether faster species is the birdcage plant, which inhabits Californian sand dunes. When its location becomes exposed, it shifts at great speed to another one with the assistance of wind — and it is this that allows many forms of vegetation to distribute their seeds. While not strictly a plant, the spores of fungi are also spread in a similar fashion. One of the most successful (and intricate) flowers to use the wind is the dandelion, whose seeds travel with the aid of 'parachutes'. They are needed to travel miles away from their parents, who are too densely packed to allow any new arrivals. Trees have the advantage of height to send their seeds further, and the cottonwood is shown as a specialist in this regard. The humidity of the tropical rainforest creates transportation problems, and the liana-species Alsomitra macrocarpa is one plant whose seeds are aerodynamic 'gliders'. Some, such as those of the sycamore, take the form of 'helicopters', while others, such as the squirting cucumber release their seeds by 'exploding'. Water is also a widely used method of propulsion. The tropical sea bean Entada gigas has one of the biggest fruits of all plants and is dispersed by water streams. However, most plants use living couriers, whether they be dogs, humans and other primates, ants or birds, etc., and to that end, they use colour and smell to signify when they are ripe for picking.

More than 90% of all crashes have a human driver as the cause. So if you want to solve traffic fatalities, the best solution is driverless vehicles. It's an ambitious goal, but only possible because of the recent breakthroughs in deep learning. Artificial intelligence is one of those key pieces that has made it possible now to do driverless cars where it wasn't possible ten years ago. For computers, until very, very recently, to do even the most basic visual tasks, like seeing a picture of a person and knowing that it's a person was remarkably hard. That's obviously fundamental to being able to understand the world around you with the sensors that you have. And we've made also gigantic strides in being able to perform complex tasks.

A ground-breaking new trial at Imperial College London sees, for the first time ever under controlled conditions, a psychedelic drug tested head-to-head against a standard antidepressant as a treatment for depression. The film follows a pioneering team of scientists and psychotherapists as they compare the effects of psilocybin (the active ingredient of magic mushrooms) with an antidepressant on a small group of participants with clinical depression.
This is scientific research at its most cutting edge. With over seven million people being prescribed antidepressants each year in England alone, this drug trial is an important milestone in understanding a completely different treatment for depression. How do psychedelic drugs measure up against the industry-standard antidepressants that have been popular since the 1990s? The empirical results of the trial are explored alongside the participants’ powerful lived experience.

Giacomo Casanova has long been hailed as a lustful beast who romped his way around Europe bedding as many women as he could. He lived a life of adventure, comedy and drama and raised questions about human relationships and sexuality that are still relevant today. How has our view of him changed in these modern times? Although this reputation is well-deserved, it doesn't tell the whole story. Casanova was also a talented and charming cheat who consorted with figures across the social spectrum - from prostitutes and pimps, to kings.

The Red Sea is one of the youngest oceans in the world and one of the warmest. The northern region, with its diverse array of marine life, is one of the most visited dive locations on earth while the southern end remains virtually unexplored. It's a global hotspot for marine biology and an important trade route throughout human history, linking the trade goods of India and the Far East with the markets of Egypt and Europe.