Ferns differ from flowering plants in that they reproduce via spores, not seeds. The spores are borne on what are called fertile fronds (leaves). Ferns have two kinds of fronds, fertile and sterile. Sterile fronds lack spores, are green, and do most of the photosynthesizing.
The fertile part of a fern can be very similar to, or totally unlike, the sterile part, depending on the genus or species. Some ferns have sori (tiny clusters of sporangia, which contain the spores), commonly referred to as “fruit dots,” on the back of the blade of their fertile fronds. Wood ferns have this arrangement, and the two types of fronds (fertile and sterile) are so similar it’s hard to tell the fertile (spore-bearing) from the sterile fronds without looking at the backs of the fronds. Others, such as Christmas Ferns, bear their sori on just a portion of the frond (tips). And still others, such as some of those pictured, have fertile fronds that are completely separate from the sterile fronds.
When the difference between the fertile and sterile fronds is noticeable, the plant is called “dimorphic”. In some ferns this dimorphism is extreme. Cinnamon, Royal and Sensitive Ferns all have separate fertile fronds that are not green and only bear sporangia. Interrupted Fern spores are borne in sporangia that appear in the middle of stalks that also bear green pinnae. The spores of all of these species of ferns are currently maturing and dispersing, which can be observed by lightly tapping a fertile frond.
(The next Naturally Curious post will appear on Thursday, June 23rd .)
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