Hand in hand with the reduction in size of single sporangia are seen more and more complex aggregations of sporangia known as sori. The meristematic area—the region of new cell growth—that produces them may continue its activities over a number of weeks, producing sporangia of all ages, older ones being pushed aside as new ones mature in their turn. When sori develop on the leaves of house ferns, they are often mistaken for tiny insects (young stages) or a fungus disease (older stages) rather than recognized as organs necessary for the normal reproduction of the plant.
The stages in progressive evolution of sori can be depicted as follows: (1) simple clusters of sporangia, these more or less coalesced (family Marattiaceae) or separate (Gleicheniaceae), all of them maturing at the same time, (2) gradate clusters of sporangia, the outermost ones maturing first, the innermost last, and (3) mixed clusters of sporangia, all ages present, the younger ones arising from the same meristematic zones as the older ones. The adaptive significance of this change is probably related to the duration of spore production, the mixed character of the more advanced sori extending the period beyond that of solitary sporangia or of simple, simultaneously maturing sori.
Sporangia and especially sori have traditionally provided the most important characters for fern classification. Indeed, many unrelated ferns were once classified together because of what are now believed to have been coincidental convergences in soral structure. Between one-half and two-thirds of the species of ferns have one or another of the following six soral arrangements: (1) A linear arrangement of sporangia along veins, avoiding the leaf area between the veins, is found in many fern genera, especially in the genus Pityrogramma. (2) A line of sporangia along the leaf edge, protected usually by a rolled-over and modified laminar margin, is represented by Pteris. (3) Round and naked sori (i.e., without an indusium) are found in Polypodium. (4) An arrangement of large sori that usually expand over the entire undersurface of the blade or pinna is represented by Acrostichum. Such sori probably arose by the fusion of smaller clusters of sori. Of the many arrangements of indusiate sori (i.e., sori that are protected by indusia, or special scalelike structures), two of the most widespread are (5) a linear or oblong sorus along a vein covered from one side by a narrow indusium, which is represented by Asplenium, and (6) a sorus that is round but covered with a kidney-shaped or shieldlike indusium, which is represented by Dryopteris.
Leave a Reply