Our Native Ferns and Their Allies 6th ed/Ch 3
|FRUCTIFICATION IN FERNS.|
|Fig. 1. — Enlarged section through a sorus of Polypodium falcatum Kellogg, showing the stalked sporangia.|
26. Spores and Sporangia. — In the flowering plants (Spermatophytes) seeds are produced by a complex process involved in pollination, the growth of the pollen tube, and the sexual process which results in the embryo of the new plant. The Ferns, on the contrary, produce no flowers. Instead of seeds developed from fertilized ovules, minute spores are produced asexually, from which new ferns are developed by a peculiar process of germination very unlike that of flowering plants. These spores are collected in little sacs known as sporangia or spore cases. The sporangia in the true ferns (Polypodiaceæ) are collected in little clusters on the back of the frond, or are variously arranged in lines along the veins or around the margins (Fig. 1). These clusters of sporangia are called sori, and may be naked, as in Polypodium, or provided with a special covering known as the indusium, as in Dryopteris (Fig. 8). The various forms of the sori and indusia serve as the basis for classification into genera and tribes, while each sub-order has its peculiar form of sporangia.
|Fig. 2. — Sporangium of Polypodium vulgare, L., discharging its spores. Much enlarged.|
27. In the Polypodiaceæ the sporangia are more or less completely surrounded with a jointed vertical ring or annulus, and at maturity burst open transversely by the straightening of the annulus and discharge their copious spores (Fig. 2). The clusters of sporangia are said to be marginal, intramarginal, or dorsal, according as they have their position at the margin or more or less remote from it. They may be roundish, oblong, or linear in shape, or arranged in variously forking lines, or may even be spread in a stratum over the entire under surface of the frond. They are called indusiate or non-indusiate according as they are covered or naked; and the indusia may be inferior (attached below the sorus), as in Woodsia (Fig. 9), or superior, as in Dryopteris (Fig.8), or of various intermediate methods of attachment.
|Fig. 3. — Enlarged sessile sporangium of Trichomanes radicans, Swz.|
|Fig. 4. — Sporangium of Schizæa pusilla Pursh, showing the apical ring. Much enlarged.|
28. In the other families of Filicales the sporangia are variously arranged. In the Hymenophyllaceæ or filmy ferns the flattened sporangia are sessile along a filiform receptacle, and are surrounded with a complete transverse annulus. At maturity they open vertically (Fig. 3). In the Schizæaceæ the sporangia are ovate, surrounded at the apex by a complete annulus, and open by a longitudinal slit (Fig. 4). In the Osmundaceæ or flowering ferns the sporangia are larger, globose, and naked, with the mere trace of a transverse annulus, and open longitudinally.
The various methods of fructification can be best understood by describing the peculiarities of the various genera in regular succession and noting the variations occurring in the sections or sub-genera. By this means we will arrive at a better understanding of the principles of fern classification as discussed in a future chapter. As the subject of venation is closely connected with that of fructification, it will be treated in the same connection.
29. Acrostichum. — In this genus the sporangia are spread in a stratum over the under surface of the upper pinnae in our solitary species, but in some exotics they cover portions of the upper surface as well. There is no indusium.
30. Polypodium (Fig. 1). — Formerly all ferns agreeing in the possession of roundish naked sori were placed in this genus notwithstanding the fact that the venation was widely different; it seems more logical to regard some of these sections as genera.
In § Eupolypdium the veins are free, yet are occasionally known to unite, thus indicating a tendency to vary toward the next section. The sori are generally found at the end of a free veinlet.
In § Goniophlebium the veins unite near the margin, forming large areolae, each containing a single free veinlet which bears the sorus at its end. A tendency to variation is seen in P. polypodioides, whose veins are free, as well as in P. Californicum in which they are often partly free.
30a. Phlebodium. — In this genus ample areolæ are next the midvein, and frequently in one or more secondary rows, each bearing a single sorus at the junction of two or more veinlets. A large number, however, bear the sori at the end of a single veinlet. From the fertile areolae to the margin the veins anastomose more copiously.
30b. Campyloneuron has areolæ, each usually bearing two sori; they are found between the parallel primary veins which extend from the midrib to the margin.
31. Gymnopteris. — In this genus the sori follow the course of the veins, and consequently vary with the venation, being simple, forked, pinnated, or anastomose with each other. The sori are non-indusiate.
32. Notholæna. — In the cloak-ferns the sori are marginal, and provided with no indusia. This genus is linked very closely to Gymnopteris on one hand and to some species of Cheilanthes on the other. From the latter it is separable only by the absence of the marginal indusium; the two are likely to be confounded by beginners.
33. Cheilogramma has simple fronds, the fructification in a continuous sub-marginal line near the apex of the frond.
|Fig. 5. — A segment of Adiantum showing the sori covered by indusia formed by reflexions of the margin of the frond — From Le Maout and Decaise.|
|Fig. 6. — Pteris longifolia L. Enlarged segment of pinna, showing the vein-like receptacle under the marginal indusium.|
34. Vittaria. — This peculiar genus occupies a somewhat intermediate position between the indusiate and non-indusiate genera, and while usually associated with the latter has considerable claim to be ranked with the former. The fronds are narrow and grass like, bearing the sporangia in an intramarginal groove, often more or less covered by the inrolled edge of the frond. The venation is very obscure.
35. Adiantum (Fig. 5). — The maidenhairs have a peculiarly smooth foliage, and usually possess no midvein. The veins are usually flabellate, and after forking one or more times bear the sori at their extremities. The margin of the frond is reflexed, thus forming an indusium which bears the sporangia on its under surface.
36. Pteris (Fig. 6). — In this genus, sori now excluding the common brake, the otherwise free veins are united by a filiform receptacle which bears the sporangia. This continuous marginal line of fructification is covered by a membranous indusium formed of the margin of the frond.
37. Cheilanthes. — The lip-ferns found within our limits are unequally divided among four sections, all agreeing in bearing the sori at or near the ends of the veins, covered by an indicium formed of the margin of the frond.
In § Adiantopsis the indusia are distinct, and confined to a single veinlet. One of our species varies from the typical species of this section, and has even been assigned to a separate genus.
In § Eucheilanthes the indusia are more or less confluent but not continuous, usually extending over the apices of several veinlets.
In § Physapteris the ultimate segments are bead-like, and the indusium is continuous all round the margin.
§ Aleuritopteris has the fronds farinose below, and includes a single species somewhat doubtfully assigned to cur limits.
38. Cryptogramma has dimorphous fronds, the margins of the fertile being closely rolled toward the midvein, thus covering the confluent sori. At maturity these open flat in order to discharge the spores.
39. Pellæa has representatives of three sections within our limits, all agreeing in possessing intramarginal sori, which finally became confluent and form a marginal line covered by an indusium formed of the margin of the frond.
§ Cheiloplecton includes herbaceous species with visible veins and broad indusia.
§ Allosorus includes coriaceous species having wide indusia, while § Platyloma includes species similar in texture, but with extremely narrow indusia and broad segments.
40. Ceratopteris is an anomalous genus from southern Florida, having a few sori arranged on two or three veins parallel to the midvein, and covered by the broadly reflexed margin of the frond. It properly forms the type of a family.
|Fig. 7. — Struthiopteris spicant. Enlarged section of the contracted fertile pinna, showing intramarginal indusium.|
41. Struthiopteris (Fig. 7) is intermediate between those genera in which there is an indusium formed of the revolute margin of the frond and those in which the indusium is remote from the margin. Our single species has dimorphous fronds, free veins, and the fructification in a broad band next the midvein, covered by acontinuous and distinctly intramarginal indusium. This genus closely resembles the next in general habit, and is sometimes united with it.
42. Blechnum. — In this genus the sori are linear and near the midvein, and are covered by a membranous indusium which is fixed at its outer margin, bursting at its inner margin when the sporangia are mature. A single representalive is found within our limits.
43. Woodwardia. — Three Species of chain-ferns occur within our limits, and each represents a distinct section based on the methods of venation. All have oblong or linear sori more or less sunken in the frond, covered by special lid-like indusia bursting at their inner margins, and arranged in chainlike rows near the midvein, thus giving the popular name to the genus.
§ Euwoodwardia has uniform fronds and veins forming at least one series of areolae between the sori and the margin.
§ Anchistea has also uniform fronds, but with free veins from the sori to the margin while § Lorinseria has dimorphous fronds, and the veins everywhere uniting to form areolae, as in the sensitive-fern (Onoclea sensibilis).
44. Asplenium. — The numerous species of spleenworts are closely related to each other in their methods of fructification, but differ widely in the form, texture, and cutting of their fronds. The sori are placed on the upper side of an oblique vein (sometimes crossing it in § Atheyrium), and covered by an indusium of the same shape attached by its edge to the fruiting vein and opening toward the midvein. In some species part of the indusia are double. The veins are free in all our species. In § Euasplenium the sori are straight or slightly curved; in § Atheyrium they are often curved, even horseshoe shaped; and frequently cross to the outer side of the fruiting vein.
45. Phyllitis bears the linear sori in pairs, one from the upper side of a veinlet and its mate from the lower side of the next. The indusia are attached by their edges to the veins, and folding toward each other appear like a double indusium covering a single sorus. The veins extend nearly at right angles to the midvein, are free, and usually forked.
46. Camptosorus. — The walking-leaf has oblong or linear indusiate sori, which are irregularly scattered and borne partly on veins parallel to the midvein, and partly on those that are oblique. Those near the midvein are single, those toward the margin are often approximate in pairs and often form crooked lines. The veins are everywhere copiously reticulated.
47. Phegopteris. — In this genus the sori are round and naked as in Polypodium, with which this genus was formerly united. The sporangia spring from the back of the veins instead of the apex, as in the latter genus, and the veins are free except in the § Goniopteris, in which they are more or less united.
|Fig. 8. — Underside of a fertile segment of Dryopteris filix-mas with eight sori. i, the indusium. Magnified. (After Sachs.)|
48. Dryopteris is largely represented in our limits by two well marked groups which it is best now to regard as distinct genera, and two others with characters scarcely less distinct, containing each a single species. In all the sori are roundish, and borne on the back of the veins or rarely at their apex. In Dryopteris the indusium is cordato-reniform or orbicular with a narrow sinus. This at first covers the sorus and is attached by its margin, but later bursts away at the margin but remains attached at the sinus. In some species in this section the indusium becomes shrivelled before the fruit matures, and in this condition might be mistaken for a non-indusiate species (Fig. 8).
In Polystichum the indusium is orbicular and peltate, being fixed by the centre; the veins are free, as § Nephrodium.
49. Nephrolepis has roundish sori borne at the apex of the upper branch of a free vein, near the margin of the frond. The indusia are usually reniform, fixed by the sinus or base, and open toward the margins of the pinnae.
50. Filix. The small bladder-ferns take their popular name from the delicate, hood-like indusium which is attached by its broad base on the inner side of the roundish sorus and partly under it. Later this is thrown back and withers away. The veins are free, and the fronds have the aspect of species of Dryopteris, but are usually more delicate in texture.
51. Onoclea. — Two quite dissimilar species have unfortunately been united under this name, which best form two genera. Both have dimorphous fronds, the margin of the contracted fertile frond being strongly revolute, and concealing the fruit. Matteuccia has necklace-shaped pinnae, crowded confluent sori, and free and simple veins. Onoclea has panicled berry-shaped pinnules and copiously anastomosing veins.
52. Woodsia (Fig. 9) has roundish sori borne on the back of the veins, with the indusia attached beneath the sporangia and flat and open, or early bursting at the top into irregular laciniae or lobes. In § Euwoodsia the indusia are flat and open from an early stage, with their cleft and ciliate margins concealed under the sori. In § Hypopeltis the indusium is more conspicuous and encloses the sporangium at first, but soon bursts at the top, forming several jagged lobes.
- Catalogue of the Davenport Herbarium, p. 8.