Sorus
(from soros, a heap). The name given to each of the little dark spots so commonly seen on the back of full-grown Fern-fronds or crowded on specially modified pinnae or fronds. Seen through a lens, these spots prove to be made up of a crowd or heap of small boxes or Sporangia (which see), each filled with Spores (which see). The sorus may be bare, as in Polypodium, or it may be covered with a membrane, called the indusium. This covering in some, e.g., Asplenium is fixed by one edge along the side of the Sorus; in others, e.g., Aspidium, it is fixed by the middle, so as to resemble a shield; in others, e.g., Hymenophyllum and Trichomanes, it forms a cup around the base of the Sorus; and there are many other less important variations in form. The Sori are situated on the veins, or at their ends along the edges of the fronds. Differences in the arrangement of the Sori, and in the structure of the indusia, are much employed in distinguishing the numerous genera of Ferns from one another.