Bryophyta

Plants that could grow above Thalloid organisms would have a distinct adaptive advantage as they could intercept light and shade out their competitors.

This requires the production of Strengthening tissues and Conducting tissues.

The Mosses (Bryophyta) have leafy, vertical shoots. They use the same strengthening and conducting tissues that were found in the Liverworts but they produce them in sufficient quantities to achieve verticality. These are still small plants which rarely top 10 cm.

The most complex species had Photosynthetic and Subterranean stems. The latter are root-like and produce hairs called Rhizoids.

These are still Small Plants reaching only a few cm. in height but they
tower over the Hepatophyta & Anthocerophyta.

Many species are terrestrial but a significant number are epiphytes. A few are aquatic or semi-aquatic.

They typically grow in wet areas but some can grow in extremely cold and dry environments where they are Pioneers.

They can have simple, minute Leaves but some species have comparatively complex Leaves which have a Nerve that has conducting & strengthening tissues.

The Bryophyta (Moss) have species that grow Vertically. This requires the development of strengthening & conducting Tissuess . The most complex species have Erect Photosynthetic and Subterranean Stems. The latter are Root-like and produce hairs called Rhizoids.

They can have comparatively complex Leaves which have a Nerve (Midrib) that has conducting & strengthening tissues.

Bryophytes produce Xylem-like Hydroids, Phloem-Like Leptoids and Sclerenchyma-like Sterids. These are present in the Central Strand of the Stem and in Leaves as well. These specialized cell types are far more developed in the Bryophyta compared to the Hepatophyta (Liverworts).

In some cases the Chlorenchyma can be complex and have prominent air spaces (Aerenchymaa). The small intercellular spaces between the Photosynthetic Cells can Hold Water due to capillary action. However, these leaves do not have an upper Epidermis to protect the Chlorenchyma from desiccation.

Diagram of a Complex Moss	A moss that has relatively broad Leaves
Pogonotum (a local Genus) has complex Leaves.	Moss Leaf with a prominent Nerve
Polytrichum Stem Cross Section. Conducting and Support Cells are typically found in the Central Strand.	Cross Section of a Polytrichum Leaf. Note the specialized Photosynthetic Parenchyma that has a lot of intercellular air spaces. It also has a Nerve that contains conducting and support tissues.
The Central Strand of this species contains Hydroids that conduct water and Sterids which provide structural support. These Hydroids do NOT have thick cell walls. The Leaf Traces represent branches from the Central Strand that connect with Leaf Nerves.	The differences between Hydroids and Sterids is clear in this SEM Image. Hydroids are analogous to Tracheary Elements & Sterids are analogous to Sclerenchyma Fibers. Some Hydroids have thick, multilayered Secondary Cell Walls and resemble Tracheary Elements. However, this is the exception rather than the rule.
Leptoids are cells specialized for the Transport of Sugars. They are analogous to Sieve Elements. Sterids are absent. Leptoids have Callose which is typically found in Sieve Elements of other land plants.
Hydroids do NOT have the type of Secondary Cell Wall Thickenings which are typical for Tracheary Elements of other land plants. *The Equisetum* Tracheid (left) has the kind of Secondary Walls that are typical for Tracheids in general. The Moss Hydroid (right) lacks the Secondary Wall thickenings. However it has the same overall shape of a tracheid. Experiments have shown that Water moves faster through Hydroids compared to other cells. Thus, they act like Tracheids!**
Leptiods and Hydroids are Elongated Cells with Overlapping End Walls. This is consistent with their function as conducting cells.	Leptoids stained to reveal Callose (bright areas). Sieve Elements contain Callose and it is rarely found in other cell types.