In seedless vascular plants, the sporophyte is the longer-lived, larger, leafy generation. This trend of sporophyte dominance throughout the evolutionary timeline of plants leads to continually smaller, less complex gametophytes. Gametophytes of this group are seldom seen. They are small and thalloid.
In Lycopodium, the gametophyte grows from a homospore and is bisexual, producing both antheridia and archegonia.
Figure \(\PageIndex{1}\): A preserved Lycopodium gametophyte. The flat thallus extending out to the right is the gametophyte. The sporophyte emerges from its left side, the root system developing downward and shoot system developing upward. This gametophyte would normally be green and photosynthetic, but the pigments were lost in the preservation process. Photo by Curtis Clark, CC BY-SA 3.0, via Wikimedia Commons
Sporophyte Morphology
Sporophytes branch dichotomously and have true roots and leaves due to the presence of lignified vascular tissue. The leaves, called microphylls, have a single, unbranched vein of vascular tissue. Note: The term microphyll, confusingly, is not an indication of the size of the leaf.
Figure \(\PageIndex{2}\): A Lycopodium sporophyte grows across a bed of feather mosses on the forest floor. Many branches stick upright and may develop strobili. Photo by Maria Morrow, CC-BY 4.0.
Figure \(\PageIndex{3}\): A closer view of the Lycopodium sporophyte shown above. The branches occur in Y-formations, showing dichotomous branching. There are many small, thin leaves (microphylls). Each one only has a single vein of vascular tissue, though this is not observable in this image. Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{4}\): A cross section through a Lycopodium microphyll. There is a single vein of vascular tissue in the center and the leaves are now many cell layers thick (unlike most bryophytes). Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{5}\): A cross section through a Lycopodium root. There is vascular tissue organized in the center with three columns of xylem (A) and phloem (B) between them. Photo by Maria Morrow, CC-BY 4.0.
Asexual propogation can occur via an underground stem called a rhizome. Homospores are produced in a structure called a strobilus that is produced at the end of a branch. A single plant can have many strobili.
Figure \(\PageIndex{6}\): A longitudinal section of a Lycopodium strobilus, shown horizontally, and labeled as follows: A) sporophyll, B) sporangium, C) spores, D) cone axis. The sporangia and spores all look approximately the same size and shape. Each sporangium is attached to a leaf (sporophyll). These are arranged around a central axis. Scale bar represents 1.5mm Photo by Jon Houseman, CC BY-SA 3.0, via Wikimedia CommonsFigure \(\PageIndex{7}\): A closer view of a Lycopodium sporangium. Notice that the spores (homospores) are all approximately the same size in these sporangia and those shown in Figure \(\PageIndex{6}\). Labels are as follows A) sporangium, B) spores, C) sporophyll. Photo by Maria Morrow, CC-BY 4.0.
