2: Early land plants
1. The phyla below belong to the earliest vascular plants -
a. Psilophyta; only 2 genera remain, mostly tropical species - this simple plant is a dichotomous stem with no roots or leaves. However the underground gametophytic stage is the minor stage, with the larger sporophytic stage dominating and they do have true vascular tissue.
b. Lycopods: they are now a bare shadow of what they once were- although 1000+species exist today, in the Carboniferous forests they were impressive parts of the landscape.
Gains: They have true leaves and roots though their leaves are microphylls Spores were moved up in the air on sporophylls- concentrated regions of sporangium on stem or leaf like structures.
Some of species are homosporous, others hetersporous. In the heterosporous species Megaspores are formed which germinate into-----> female gametophytes, along with micro spores which germinate into ------> male gametophytes.
c. Horsetails: also a big part of Carboniferous period, few species remain today.
d. Ferns: 12,000 species still exist the major evolutionary gain is the production of Megaphylls which we call fronds. No longer 1-veined.
2. The major changes included:
A. Development of true phloem and xylem and along with this, the development of true leaves, roots and stems ( by definition they are not true without a supporting vascular system)
In the diagram to your right a potential model of leaf evolution:
a. dichotomous branching
b. planation = 3d plane moved and flattened ( imagine flattening down a triangle)
c. webbing with chlorophyll ladened cells.
B. Role reversal: The gametophyte (1N) which was the main life stage was taken over by the sporophytic stage (2N). What is the significance of this change?