Lab 10: The Land Plants - An Evolutionary Overview  
Sporophyte Dominance 

(A) Psilophyta - "whisk ferns"

    Once regarded as the most primitive extant vascular plants, the two remaining genera of Psilophyta are now thought to represent a highly reduced lineage of ferns. Nevertheless, many of the characteristics of Psilophyta resemble those of the earliest vascular plants, as seen in the fossil record.

"Primitive" Characteristics of Psilophyta

1) Dichotomous branching. 

2) Absence of leaves

3) Little differentiation of stem and root morphology.

Characters Uniting Psilophyta with Ferns

1) Fused sporangia (synangia). 

2) Fern-like flagellate sperm cells. 

3) Vascular tissue in gametophyte

4) DNA sequence data.

    The two genera of Psilophyta, Psilotum and Tmesipteris, are mainly tropical epiphytes, but the range of Psilotum extends as far north as southern Florida. Observe the living examples of Psilotum nudum, as well as C&V fig. 6.23-6.27. The plant body consists of an underground rhizome (stem) from which arise the upright green branches.

  1. Note the regular, forked branching pattern in the above-ground portion of the plant. This is termed dichotomous.
  2. Can you find any structures that might represent leaves?
  3. Can any other structures be observed? What are they?

Psilotum nudum
dichotomous branching
Psilotum sporangia
Psilotum nudum: aspect.
Dichotomous branching in Psilotum.
Sporangia of Psilotum

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