They serve to conduct water through the thallus, the assimilating parts of which are in these forms often raised above the soil and are comparatively remote from the rhizoid-bearing (water-absorbing) region.
In the compact thalli a secondary development often takes place by the growth of rhizoid-like internal filaments.
In C, lying horizontally, an additional cell has been cut off between rhizoid and spore.
In addition to the cell types described, it is a very common occurrence in these bulky forms for rhizoid-like branches of the cells to grow out, mostly from the cells at the periphery of the medulla, and grow down between the cells, strengthening the whole tissue, as in the Rhodophyceae.
It corresponds in function with the rhizoid of i Bryophyte.