All true ferns come out of the ground head foremost, coiled up like a watch-spring, and are designated as "fiddle-heads," or crosiers. (A real crosier is a bishop's staff.) Some of these odd young growths are covered with "fern wool," which birds often use in lining their nests. This wool usually disappears later as the crosier unfolds into the broad green blade. The development of plant shoots from the bud is called vernation (Latin, ver meaning spring), and this unique uncoiling of ferns, "circinnate vernation."
VEINS
The veins of a fern are free, when, branching from the mid-vein, they do not connect with each other, and simple when they do not fork. When the veins intersect they are said to anastomose (Greek, an opening, or network), and their meshes are called arèolæ or áreoles (Latin, areola, a little open space).
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EXPLANATION OF TERMS A frond is said to be pinnate (Latin, pinna, a feather), when its primary divisions extend to the rachis, as in the Christmas fern (Fig. 1). A frond is bipinnate (Latin, bis, twice) when the lobes of the pinnæ extend to the midvein as in the royal fern (Fig. 2). These divisions of the pinnæ are called pinnules. When a frond is tripínnate the last complete divisions are called ultimate pinnules or segments. A frond is pinnátifid when its lobes extend halfway or more to the rachis or midvein as in the middle lobes of the pinnátifid spleenwort (Fig. 3). The pinnæ of a frond are often pinnátifid when the frond itself is pinnate; and a frond may be pinnate in its lower part and become pinnátifid higher up as in the pinnátifid spleenwort just mentioned (Fig. 3). The divisions of a pinnátifid leaf are called segments; of a bipinnátifid or tripinnátifid leaf, ultimate segments. | |
| [Illustration: Fig. 1] | |
| [Illustration: Fig. 2] | [Illustration: Fig. 3] |
[Illustration: Fig. 4]
SPORÀNGIA AND FRUIT DOTS
Fern spores are formed in little sacs known as spore-cases or sporángia (Fig. 4). They are usually clustered in dots or lines on the back or margin of a frond, either on or at the end of a small vein, or in spike-like racemes on separate stalks. Sori (singular sorus, a heap), or fruit dots may be naked as in the polypody, but are usually covered with a thin, delicate membrane, known as the indusium (Greek, a dress, or mantle). The family or genus of a fern is often determined by the shape of its indusium; e.g., the indusium of the woodsias is star-shaped; of the Dicksonias, cup-shaped; of the aspleniums, linear; of the wood ferns, kidney-shaped, etc.
In many ferns the sporangia are surrounded in whole or in part by a vertical, elastic ring (annulus) reminding one of a small, brown worm closely coiled (Fig. 4). As the spores mature, the ring contracts and bursts with considerable force, scattering the spores. The spores of the different genera mature at different times from May to September. A good time to collect ferns is just before the fruiting season. (For times of fruiting see individual descriptions or chronological chart on page 220.)