In this article we will discuss about the structure of Porella with the help of diagrams.
External Structure of Porella:
The gametophyte of Porella is flat, dorsiventral and foliose (Fig. 6.21 A). The prostrate stem or axis is bi- or tripinnately branched. There are three rows of leaves arranged on the stem, two dorsal rows and one ventral row. The dorsal rows form the lateral leaves and the leaves of the ventral row are called amphigastria (Fig. 6.21 B). Leaves are devoid of midribs.
The dorsal leaves are bilobed. The upper anterior lobe, called antical lobe, is larger, usually ovate with rounded apex; while the lower posterior lobe, called postical lobe or lobule, is much smaller and narrower with acute apex.
The antical and postical lobes can be clearly seen when the thallus is viewed from the ventral side (Fig. 6.21 B). The dorsal leaves are closely overlapping and show incubous arrangement i.e. the lower edge of each leaf is covered by the upper edge of the next leaf below. This arrangement can be seen when viewed from the dorsal side.
The amphigastria on the ventral surface are small leaves resemble the postical lobes of the dorsal leaves, but are broader with frequently decurrent bases.
A large number of scattered rhizoids of smooth-walled type arise from the ventral surface of the stem. The main function of rhizoids is to attach the thallus to the substratum. The absorption of water and minerals is taking place primarily through the leaves and stem.
Internal Features of Porella:
Stem:
The stem shows little tissue differentiation. The T.S. of the mature stem shows two distinct regions, the cortex and the medulla (Fig. 6.22A). The cortex is a 2-3 layered zone, consists of thick-walled parenchymatous cells, while the medulla is composed of thin-walled, elongated cells.
Leaf:
The leaves are very simple in configuration. Each leaf is composed of a single layer of isodiametric parenchymatous cells containing many chloroplasts (Fig. 6.22B). In a few species, leaves contain oil cells.
