Non-vascular plants

We do not know if non-vascular plants evolved from multicellular green alga, and then gave rise to primitive vascular plants, or as the molecular data indicate, non-vascular and vascular plants land plants evolved separately from an algae ancestor. The non-vascular plants include the modern mosses (phylum Bryophyta), liverworts (phylum Hepatophyta), and hornworts (phylum Anthocerophyta).   These plants are small and low-growing for two reasons.  First, their lack of vascular tissue limits their ability to transport water internally, restricting the size they can reach before their outermost portions dry out. They do have cuticles which block some water loss with stomata for gas exchange. Secondly, they exhibit alternation of generations, involving separate plant forms., The gametophyte form depends on swimming sperm to fertilize other the eggs of other plants.  This necessitates a film of water for the sperm to swim through, which is easier for a small low-growing plant to achieve. 

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Figure 10 Non vascular plants. 

A. Liverwort with insert showing the short lived sporophyte state. 

B. Moss showing both stages; sporophytes are generally produced only for a short time in the spring.  

C. The body or thallus of a hornwort resembles that of a liverwort, but the two can be distinguished by their spore cases. Hornworts produce green spikes (horns) that mature and slowly split lengthwise from the tip down,  making and releasing their spores slowly.

The major difference between  this group and the others is the life cycle. In all three groups of non-vascular plants the gametophyte dominates the life cycle. Note that in this group,  structures that produce sperm are known as antheridia and the structures that produce eggs are known as archegonia. 

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Figure 11 Life cycle of a non-vascular plant

The sporophyte portion of the life cycle appears infrequently and is attached to the gametophyte. The sporophyte tends to be much taller than the gametophyte, extending a tall stalk upwards so that the spores it produces can be carried by the wind. The life cycle of a typical moss is illustrated below.  This life cycle is similar to those of the liverworts and hornworts. 

View animation of moss life cycle