Some mosses also have solute-conducting leptoids surrounding a central bundle of hydroids. Leptoids are elongate cells that have nuclei and living protoplasts and thus closely resemble the most generalized phloem cells of some vascular plants. Hydroids may also be found in moss sporophytes , but leptoids have been found only in the sporophytes of a few genera. P-type cells characteristics of Psilophyton and many common living plants. The fossil record of early land plants preserves a variety of other conducting-tube forms.
Some tubes are smooth and lack ornamentation.
This conducting cell type is typical of zosterophylls and early lycopsids. The P-type cell has scalariform pitting typical of most modern vascular plants. The term homology was first introduced by zoologist Sir Richard Owen in 1843. Homology refers to structures or organs that have evolutionary correspondence, regardless of their current function. The homology of structure is based on similarities in morphology or developmental origin.