Polyethylene is a very common thermoplastic with low strength and rigidity, and a low melting point. Its relative flexibility makes it perfect for large thin parts like leaves and stems. On the left in the photo above are high-density polyethylene (HDPE), while the right two parts are low-density polyethylene (LDPE): the lower the density, the softer it is. PE is a very versatile plastic — the thin plastic in old LEGO store shopping bags is LDPE, and some carry cases that have a living hinge are HDPE. In 2018, LEGO started sourcing their PE from 98% sugarcane (the two small leaves pictured), as part of their long-term sustainability plan. Although the source is different, the end product is, in theory, identical.
[Original post and comments on Flickr.]