Caia is a genus of small fossil plants of Late Silurian age (around ). The diagnostic characters are naked parallel-sided axes branching isotomously, terminating in vertically elongate sporangia

Sporangium

A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...

 (spore-forming organs) which bear spinous emergences particularly at the distal ends. Spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s are trilete and retusoid. The only known species is from Hereford

Hereford

Hereford is a cathedral city, civil parish and county town of Herefordshire, England. It lies on the River Wye, approximately east of the border with Wales, southwest of Worcester, and northwest of Gloucester...

, England.

Cladistic analysis suggests that the genus may belong to the Horneophytopsida

Horneophytopsida

Horneophytopsida is a class of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue, found from the Late Silurian to the Early Devonian . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...

, a class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of the polysporangiophyte

Polysporangiophyte

Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation has a structure of branching stems terminating in sporangia...

s, as it lacks vascular tissue

Vascular tissue

Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...

 and has branched stems bearing sporangia. For the cladogram, see the Horneophytopsida

Horneophytopsida

Horneophytopsida is a class of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue, found from the Late Silurian to the Early Devonian . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...

article.