In their element
David Attenborough wades into a stream and collects aquatic algae to show that it is floppy and unsuitable for life on land.
It needs water to support its stem and allow it to grow upright.
The early land plants that descended from it were therefore lowly, forming flat skins as with liverworts, or cushions as in mosses.
All of them lived in wet moist places, for a very good reason.
Their ancestors - the algae - had reproduced in two ways, sexually and by budding.
The sexual method involved the sex cells swimming through water to find one another and fuse.
Modern mosses have retained much the same method and it is this that has kept them tied to water.