Small, non-vascular plants known as bryophytes include mosses, liverworts, and hornworts.
Because they act as an important buffer system for other plants that coexist with and benefit from the water and nutrients that bryophytes absorb, they are crucial to the regulation of ecosystems.
Bryophytes are small because they have limited transportation options for gases, water, and other nutrients.
So little that even the largest moss can only reach a height of 50cm.
Larger colonies of bryophytes do exist, though.
According to sources, bryophytes are limited in size due to genetically acquired features. Their size is due to their construction.
The lack of a tubular circulatory system, however, is thought to be the main reason for their diminutive size.