WB01343_.gif (599 bytes)   Mosses (Bryophyta)  WB01345_.gif (616 bytes)

The next step in Evolution was a quest to become Vertical. 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) are the firstBryoComplexMossHab-2240.jpg (12048 bytes) group to develop 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 Rhizoids.

These are still Small Plants reaching only a BryoComplezMoss200.jpg (67467 bytes)few cm in height but they tower over the Hepatophyta & Anthocerophyta.

Many species are terrestrial but a significant number are epiphytes. They typically grow in wet areas but some can grow in extremely cold and dry environments where they are Pioneers.

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

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Diagram of a Complex Moss		 BryoRrhodobryumPlantsTopCrop.jpg (65501 bytes)
A moss that has relatively broad Leaves

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Pogonotum (a local Genus) has complex Leaves.

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Moss Leaf with a prominent Nerve

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Polytrichum Stem Cross Section. Conducting and Support Cells are typically found in the Central Strand.

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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.

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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.

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The differences between Hydroids and Sterids is clear in this SEM Image. Hydroids are analogous to Tracheary Elements & Sterids are analogous to Sclerenchyma Fibers.

 

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Some Hydroids have thick, multilayered Secondary Cell Walls and resemble Tracheary Elements. However, this is the exception rather than the rule.

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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.

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Hydroids obtained from different Moss species. Their overall appearance and the localization of their pores resembles that seen with Tracheary Elements.

 BryoHydvslptColrLab.jpg (42503 bytes)

 

 

Hydroids do NOT have the type of Secondary Cell Wall thickenings which are typical for Tracheary Elements
BryoLepHygColrL-SLab.jpg (37704 bytes)
Leptiods and Hydroids are Elongated Cells with Overlapping End Walls. This is consistent with their function as conducting cells.		 PryoLeptoCalloseFluor.jpg (28859 bytes)
Leptoids stained to reveal Callose (bright areas). Sieve Elements contain Callose and it is rarely found in other cell types.

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