In this article we will discuss about the process of reproduction in porella with the help of diagrams.
Porella (Fig. 2.11 A, B) is dioecious and dimorphic. Male plants are smaller than female plants. The antheridia occur on small lateral branches (Fig. 2.11C) which are almost perpendicular to the main axis. Although the female plants are bigger than male plants, an archegonial branch (Fig. 2.11D) is smaller than a male branch.
Leaves (bracts) on an antheridial branch are close together and much smaller than leaves on a vegetative branch. Each bract bears in its axis an antheridium (Fig. 2.11E). At times, an antheridial branch forms a secondary branch which in turn produces antheridia.
The antheridia dehisce in a characteristic manner. On absorbing water the wall of an antheridium bursts open towards the apex, into a number of valves which curl backwards. The force is so great that at times individual cells separate out.
Archegonia develop at the apex of short archegonial branch (Fig. 2.11 D) of female plants.
Each archegonial branch bears about 15 archegonia, surrounded by two bracts, each one two-lobed with a dentate margin, and a dentate bracteole. The bracts and bracteole are modified leaves and an amphigastrium. These bracts and bracteole develop into a perianth which provide protection to the developing sporophyte.
A mature archegonium has a long neck which is as broad as venter. The neck is composed of five rows of neck cells enclosing 6-8 neck canal cells. The venter becomes two-layered prior to fertilization of egg to form zygote.
The zygote divides transversely to form an epibasal and a hypobasal cell, the latter remains undivided to form a haustorium or suspensor. The epibasal cell divides in an irregular manner to form an elongated multicellular embryo wherein it is difficult to demarcate the zones of foot, seta and capsule.
This is followed by differentiation of amphithecial and endothecial layers. The amphithecium cells divide periclinally and anticlinally to form 3-layered-thick wall of capsule. The endothecium divides and redivides to form the archesporial cells.
The last division of archesporial cells results in differentiation of spore-mother-cells and elater- mother-cells, which are intermingled. The spore-mother-cells become four-lobed and the nucleus undergoes reduction division to form four haploid spores. The elater-mother-cells elongate to form tapered elaters with spirally thickened bands.
A sporophyte (Fig. 2.12A) has an indistinct foot, a short seta and a globose capsule. On maturity the elongating seta (Fig. 2.12B) pushes the capsule out of calyptra, perianth and involucre. The capsule dehisces by opening along four lines into 4 valves. Hygroscopic movements of elaters aid in the dispersal of spores.
Prior to dispersal, the spores start dividing within the capsule. This multicellular structure or protonema differentiates an apical cell from which arises the gametophyte (Fig. 2.12E). The gametophyte develops a few rhizoids towards the basal region.
