Plants in the Lycophyta have erect stems as well as Stolons and Rhizomes. They are relatively large compared to Hepatophyta, Bryophyta and Psilophyta but they rarely exceed a meter in height. They can be epiphytic and pendant stems can be more than a meter in length. They have Microphylls and Roots. Branching is Dichotomous for both organs. The Apical Meristem has several "Initials" rather than a solitary Apical Cell.
We will examine the genus Lycopodium first.
SEM photo of a Lycopod SAM |
Long Section of a Lycopod SAM: Note the presence of several large "Initials" at the summit of the stem. |
These plants are most abundant in the tropics but a few can survive cold, dry environments.
While extant Lycopods are small plants with little ecological significance. Forests of tree-sized lycopods once dominated certain habitats. The most famous of these is Lepidodendron which reached heights up to 30 meters. They had secondary growth. The stems were coated with leaf bases and there appeared to be little internodal elongation. The latter implies that the trees had slow growth which may partly account for their extinction.
Lepidodendron Stem Base |
The next major step in evolution was the development of a complex leaf called a Megaphyll.