Q.What are elaters?

The Marchantia sporophyte consists of a foot, a seta, and a spore-and-elater-producing capsule — since it cannot photosynthesise independently, it remains nutritionally dependent on the parent gametophyte throughout its short life.

Marchantia is a thalloid liverwort (Bryophyta); after fertilisation of the egg in the archegonium by a motile sperm, the resulting diploid zygote develops into the sporophyte generation while still attached to (and dependent on) the parent gametophyte thallus. The Marchantia sporophyte is a relatively simple, short-lived structure, differentiated into three parts (unlike the more complex mosses):

1. Foot — a small, bulbous structure that remains permanently embedded within the tissue of the parent gametophyte (inside the archegonium/involucre). It functions purely as an absorptive/anchoring organ, drawing water, minerals and organic nutrients from the gametophyte, since the sporophyte itself has no chlorophyll-bearing photosynthetic tissue of its own and is entirely dependent (parasitic) on the gametophyte.
2. Seta — a short stalk connecting the foot to the capsule. It remains meristematic and largely undifferentiated for most of sporophyte development, but rapidly elongates just before spore release, pushing the capsule up and out of the protective involucre so that spores can be dispersed away from the parent plant. …