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Volume 7, 1874
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Art. XLVIII.—Observations on the different Modifications in the Capsules
of Mosses, with, reference to the Dispersion of their Spores.

By Captain F. W. Hutton, C.M.Z.S., etc.

Plate XXII.

[Read before the Otago Institute, 12th October, 1874.]

The fructification of mosses consists of a variously shaped capsule, or sporangium, generally placed on a stalk, which is sometimes short, but generally much elongated (Plate XXII., fig. 13). Through the centre of the capsule rises a pillar of cellular tissue called the columella (fig. 15), and it is round the columella, between it and the wall of the capsule, that the spores, or seeds, are formed.

It is evident that if these spores were liable to be all blown away together by the first puff of wind that occurred after they were ripe, or to be all knocked out by the first drop of rain that fell upon the capsule, they would have but little chance of being widely dispersed. If also the spores escaped and were blown away in moist weather they could not travel far, for they would stick to the first leaf or stone that they were blown against. If, on the other hand, there were some means of holding the spores fast in wet weather, and letting them escape when the air was dry, and then only by a few at a time, so that some might be driven off in one direction, some in another, it is evident that they would then be scattered as far and as wide as possible.

But as mosses inhabit very different stations, some living among long grass or in swamps, others on exposed walls; some on trees, others under water; some in sheltered ditches, and others on mountain peaks, different combinations will be required by different species. For example, those species that grow among long grass require a long fruit-stalk to elevate the capsule above the grass and so let it be exposed to the wind; while this would be detrimental to those mosses that live in exposed situations, and these generally have the fruit-stalk so reduced in length that the capsule is buried in and protected by the perichætial leaves, unless the species possesses some other and more perfect way of preventing its spores being blown away; and I propose to offer a few observations on the different approaches towards perfection in attaining this object that are found in various genera and species of mosses.

I have already said that the spores are contained in a capsule placed on the top of a fruit-stalk. In by far the larger number of mosses this capsule is furnished with a lid, or operculum (fig. 14), which falls off when the spores are ripe, and the mouth of the capsule is then seen to be either naked (fig. 1), or furnished with numerous teeth and cilia, called the peristome (fig. 14). The

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fruit-stalk is sometimes straight, but often curved, so that the capsule is inclined or pendulous (fig. 13), with the mouth turned downward and thus protected from rain. In some mosses the same effect is obtained by the capsule itself being cernuous, while the fruit-stalk is straight.

In order to give some method to the remarks that follow I shall divide the mosses into (1) those in which the capsule alone is used; (2) those in which the peristome is used; and (3) those in which the columella is used, combined either with the operculum or with the peristome.

1. Mosses in which the capsule alone is used.

In the lowest forms, the family Phascei, the capsule decays and bursts open irregularly so that the spores are quite exposed, except in those species where the capsule is immersed in the perichætial leaves. But it must be remarked that in Phascum cuspidatum, which is the only species in which I have been able to make the observation, the perichætial leaves open when wet and close when dry, so that the protection thus afforded can be but slight. They have, however, very few spores in a capsule, Phascum alternifolium only having about sixteen, and P. serratum about one hundred, so that it would hardly be necessary to protect so few from being all blown away together. These mosses are all inhabitants of the plains, and grow on ditch banks and other sheltered situations; but there is another family, the Andreœeœ, which lives on exposed rocks on mountains, and yet has the capsule not much more complicated than in the humble Phascei. In this family when the spores are ripe the capsule divides longitudinally into four or more parts, the upper extremities of which adhere together. These parts bend downward when dry, and allow the spores to escape through the open fissures (fig. 8b), but on the air getting damp they straighten and close up the capsule again quite tight (fig. 8a), and thus prevent the spores being blown away in wet weather.

The first step towards a more complicated state of things is when the capsule, instead of bursting through decay, is furnished with an operculum, which falls off when the spores are ripe, thus leaving them to some extent protected by the vase-like form of the capsule. Some genera of mosses, such as Braunia, Physcomitrium, and Aulacopilum, have no further provision against all their spores being knocked out at once; but in others, as Hedwigia and Leptangium, * the capsule is immersed in the leaves. In the genus Sphagnum, when the capsule is ripe, the operculum and spores are said to be driven off with violence to a distance of six or seven inches. The next step is the formation of a flexible ring of very hygroscopic cells, called the annulus (fig. 14), interposed between the mouth of the capsule and the operculum, as in Œnectangium and Calomnion, which protects in some measure the spores from

[Footnote] * Leptangium, is said by Dr. Hooker to have the mouth of the capsule closed in by a membrane. I have had no opportunity of examining it personally.

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the wind; while in some species of Gymnostomum, as well as in Cadomnion, a further provision is made by the walls of the capsule being much thickened at the mouth, which reduces the size of the opening. In most of those genera in which the apparatus for protecting the spores is more perfect this annulus falls away * after it has performed its office of throwing off the operculum from the capsule, but in the genera just mentioned it is persistent. †

In Œnectangium also the spores are very minute and numerous, which gives evident facilities for their far and wide dispersion ‡

2. Mosses in which the peristome is used.

I come now to that large division of mosses which have a series of teeth, called the peristome, arranged round the mouth of the capsule. In its most complete state this peristome consists of an external row of sixteen conical teeth, and an internal folded membrane divided above into processes sometimes with cilia between each (fig. 7). The inner peristome is, however, variously modified, and is often altogether absent. The outer peristome consists normally of sixteen teeth, which are sometimes split half way down, as in Dicranum, Fissidens, etc. Sometimes they are divided to the base so as to make their number thirty-two, as in Trichostomum and Tortula. In rare cases also two teeth are joined together so as to reduce their number to eight, as in the tropical genus Octoblepharum, while occasionally they are reduced to four, as in Tetraphis.

The peristome also is very variously developed in different mosses. In some, such as Weissia, Didymodon, Conomitrium, etc., the teeth are short and fragile and soon break off. In Trichostomum, Encalypta, etc., they are stronger; while in most genera they are strong and answer admirably their purpose of preventing the too easy escape of the spores.

These mosses can be divided into five groups in each of which the peristome is used in a different way.

(a.) The first group, which is represented in New Zealand by the genera Dicranum, Dicranodontium, Campylopus, Racomitrium, † Leucobryum, Grimmia, Ceratodon, Conostomum, and Symblepharis, have the peristome long and well formed, and the teeth stand nearly erect round the mouth of the capsule with

[Footnote] * Symblepharis is an exception.

[Footnote] † Brachyodus might be placed here, for its peristome being shorter than the annulus, it cannot in any way affect the dispersion of the spores.

[Footnote] ‡ I have not, however, been able to trace any connection between the abundance or rarity of a moss and the size of its spores. For instance, the spores of Hypnum cupressiforme and H. rutabulum are nearly twice the diameter of those of H. prœlongum and H. confertum, while H. triquetrum and H. purum hold an intermediate position.

[Footnote] † In Racomitrium aciculare the teeth are slightly hygroscopic, spreading when dry, but erect and slightly incurved when wet. I have noticed that in species a sticky substance is found amongst the spores, which is drawn out into lines like cobwebs between the teeth, and by this means the spores are often stuck on to the inner side of the peristome.

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their points touching or interlaced, so as to form a kind of cage over the mouth which protects the spores from the wind, but allows them to be blown away in small quantities at a time, and by high winds only. In Conostomum (fig. 5), which inhabits exposed rocks on mountains, this cage is further strengthened by the tips of the teeth cohering together, and this moss may be said to be the type of the group. All these mosses have strong persistent peristomes well calculated to resist the wind, consequently we find that many of them inhabit mountains or subalpine regions, and several grow upon exposed rocks, while in some species of Grimmia only is the capsule immersed in the perichætial leaves. The peristome however is not able to protect the spores from the wet, and to accomplish this the greater part have the mouth of the capsule inclined or bent downwards either through the fruit-stalk being curved, or the capsule being cernuous or drooping. I must here remark that the fruit-stalk of Campylopus is hygroscopic, being curved and burying the capsule among the leaves in dry weather, while when the air is moist standing more erect, an anomaly for which I can offer no explanation. In Seligeria recurvata the fruit-stalk straightens when dry.

(b.) In the second group, which is represented in New Zealand by Dawsonia superba and the genera Trichostomum and Tortula, the peristome consists of numerous filiform, slightly hygroscopic teeth, which are either distinct from one another, or united at the base into a membrane. In Trichostomum, and Dawsonia superba (fig. 11) the teeth are nearly straight, but in Tortula (fig. 9) they are twisted into a spiral, which in damp weather twists tighter and closer over the mouth of the capsule. The mosses in this group are preeminently inhabitants of the plains, their peristome being too weak and fugacious to withstand the boisterous winds of the mountains. Tortula possesses the strongest peristome, and consequently we often find it growing on walls and other exposed places, while Trichostomum and Didymodon generally grow on the ground or in sheltered hollows. All have the fruit-stalk elongated, and the capsule erect or nearly so.

(c.) In the third group the teeth are all more or less hygroscopic, opening when dry and closing over the mouth of the capsule when wet. In some, e.g., Phyllogonium (fig. 10) and Blindia, the movement is small and the teeth when wet never get beyond an erect position; in others, e.g., Fissidens and Cryphœa, they continue the movement until they have attained a horizontal position, and then curl their points inward; in others again, such as Orthotrichum, Zygodon (fig. 3), Eremodon and Fabronia, the movement is still further continued until their backs are adpressed against the outer sides of the capsule. In this group the peristome is beautifully adapted for preventing the spores being dispersed in wet weather, but there is nothing to prevent them being all blown away in one direction by the first strong wind, and we find therefore that they are chiefly inhabitants of the plains, and generally

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grow in forests and sheltered places. With the exception of some species of Orthotrichum the capsule is always exserted, and in Macromitrium it is sometimes much elongated. Fissidens is the only genus with the mouth of the capsule inclined or directed downward.

(d.) In the fourth group the peristome is double, the inner one being composed of a membrane more or less divided in the upper part. The outer peristome is hygroscopic, closing tight over the mouth of the capsule when moist, and spreading outward with the tips of the teeth incurved when dry. The inner peristome is not hygroscopic, but on the opening of the outer peristome the interior projections, or “trabeculæ”, as well as the points of the teeth, get entangled in the perforations and cilia of the inner peristome and drag it open, often quitting their hold with a jerk which spirts out the spores to some little distance, thus answering the same purpose as the elaters of the Hepaticœ. This group is a very large one and includes the New Zealand genera of Bryum, Cladomnion, Isothecium, Hypnum (fig. 7), Rhizogonium, Hypopterygium, Racopilum, Hookeria, and others. These mosses are chiefly inhabitants of plains and forests; in nearly all the species the capsule is inclined, while in some, e.g., Bryum and Mnium, it is pendulous (fig. 13), so that its mouth is directed straight downward. All have the fruit-stalk elongated except Fontinalis, a northern genus that lives in water. In this moss (fig. 6) the outer peristome is hygroscopic, and the inner is converted into a conical membrane regularly perforated with square holes, which prevents the spores being washed ont too quickly.

(e.) The fifth group consists of the genus Funaria alone, which stands apart from all other mosses as far as the apparatus for the dispersion of the spores is concerned. In this moss the peristome is double, the points of the teeth of the outer one being connected by a disc, which in time falls away. The inner consists of narrow teeth, which might almost be called cilia. Both are hygroscopic, and curl outward when dry after the rupture of the disc. The capsule also is pendulous on a hygroscopic fruit-stalk, which, however, when either curling or uncurling, always keeps the mouth of the capsule directed downwards, and thus protects it from the rain. We thus see that this moss combines the advantages of the first and third groups. At first its strong united peristome enables it to distribute its spores in small quantities to high winds only, while afterwards the mouth of the capsule opens wide, and allows, on the first dry day, all those spores that still remain to be blown away, and so prevents them from being wasted through failing to escape from the capsule. This is perhaps the most perfect apparatus possessed by any moss, and we cannot wonder at its almost cosmopolitan distribution.

3. Mosses in which the columella is used.

These mosses may be divided into two groups, which use the columella in connection with the operculum or the peristome respectively.

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(a.) In some European mosses, such as Gymnostomum curvirostrum, Pottia truncata (fig. 1), P, heimii, and Stylostegium cœspiticum, the operculum adheres to the columella, which elongates when the spores are ripe and lifts up the operculum. Some mosses even which have peristomes have also the operculum adhering to the columella, such as Dissodon hornschuchii, from Europe and North America; and in Climacium dendroides, also from Europe, the operculum is lifted up in dry weather, but when the air is moist the columella contracts again and closes up the mouth of the capsule.

(b.) In the European genus Cinclidotus (fig. 2), which lives in water, the columella is exserted and the filiform teeth of the peristome are twisted round it, thus forming a cage as in Fontinalis, but on quite a different principle. When the capsule gets dry or old the columella shrinks and breaks off the upper part of the peristome, thus liberating those spores that had previously failed to make their escape.

In Splachnum (fig. 12) the columella is large, exserted, and dilated at the top. In moist weather the teeth fold over the mouth of the capsule and touch the dilated apex of the columella, but in dry weather they bend back until they are adpressed against the outer side of the capsule.

In our species of Dawsonia the filaments are, as I have said, free, but in some Australian species these filaments, or cilia, are connected with the top of the columella. In Wardia, from the Cape of Good Hope, the peristome consists of an irregularly fissured membrane which adheres to the top of the columella, and Scouleria, from North America, has thirty-two laciniated teeth which are also connected with a process on the top of the columella, (Berkely).

But it is in the Polytrichaceœ that the columella is put to the greatest use for protecting the spores. In the genus Polytrichum (fig. 4) the top of the columella is spread out into a thin flat drum-like membrane that extends over the whole orifice of the capsule, and is connected at its edge with the mouth of the capsule by thirty-two or sixty-four short processes formed of many threads, like those in Dawsonia, soldered together, so that the spores can only escape through the apertures between these processes, and consequently only a few are likely to be blown out at a time. The genus Lyallia, from India, has a similar membrane over the mouth of the capsule, but at length the columella contracts within the capsule, and detaches a circular portion from the centre of the drum, thus releasing those spores which have failed to make their escape. As this membrane extends over the whole of the mouth of the capsule, it entirely excludes the rain when the capsule is erect, which position is therefore in this case better than an inclined one, and is that which nearly every species of the family assumes. The species are equally distributed between the plains and the mountains.

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