Ferns are admirably adapted to rapid dispersal over comparatively wide areas. Bower[723] estimates that in one season a Male Fern may produce about 5,000,000 spores: with this enormous spore-output are coupled a thoroughly efficient mechanism for scattering the germs and an unusual facility for wind-dispersal. When Treub[724] visited the devastated and sterilised wreck of the Island of Krakatau in 1886, three years after the volcanic outburst, he found that twelve ferns had already established themselves; the spores had probably been carried by the wind at least 25 to 30 miles. It is not surprising, therefore, to find that many ferns have an almost world-wide distribution; and, it may be added, in view of their efficient means of dispersal, wide range by no means implies great antiquity. Prof. Campbell[725] has recently called attention to the significance of the wide distribution of Hepaticae in its bearing on their antiquity; the spores are incapable of retaining vitality for more than a short period, and it is argued that a world-wide distribution can have been acquired only after an enormous lapse of time. If we apply this reasoning to the Osmundaceae among ferns, it may be legitimate to assume that their short-lived green spores render them much less efficient colonisers than the great majority of ferns; if this is granted, the wide distribution of Osmundaceous ferns in the Mesozoic era carries their history back to a still more remote past, a conclusion which receives support from the records of the rocks.
The Bracken fern which we regard as characteristically British is a cosmopolitan type; it was found by Treub among the pioneers of the New Flora of Krakatau; in British Central Africa, it greets one at every turn “like a messenger from the homeland[726]”; it grows on the Swiss Alps, on the mountains of Abyssinia, in Tasmania, and on the slopes of the Himalayas. The two genera Matonia ([fig. 228]) and Dipteris, which grow side by side on Mount Ophir in the Malay Peninsula, are examples of restricted geographical range and carry us back to the Jurassic period when closely allied types flourished abundantly in northern latitudes. Similarly Thyrsopteris elegans, confined to Juan Fernandez, exhibits a remarkable likeness to Jurassic species from England and the Arctic regions.
The proportion of ferns to flowering plants in recent floras is a question of some interest from a palaeobotanical point of view; but we must bear in mind the fact that the evolution of angiosperms, effected at a late stage in the history of the earth, seriously disturbed the balance of power among competitors for earth and air. The abundance of ferns in a particular region is, however, an unsafe guide to geographical or climatic conditions. Many ferns are essentially social plants; the wide stretches of moorland carpeted with Pteris aquilina afford an example of the monopolisation of the soil by a single species. In Sikkim Sir Joseph Hooker speaks of extensive groves of tree ferns, and in the wet regions of the Amazon, Bates[727] describes the whole forest glade as forming a “vast fernery.” In a valley in Tahiti Alsophila tahitiensis is said to form “a sort of forest almost to the exclusion of other ferns[728].” In the abundance of Glossopteris (figs. [334], etc.) fronds spread over wide areas of Permo-Carboniferous rocks in S. Africa, Australia, and India, we have a striking instance of a similar social habit in an extinct fern or at least fern-like plant.
Acrostichum aureum, with pinnate fronds several feet long, is an example of a recent fern covering immense tracts, but this species[729] is more especially interesting as a member of the Filicineae characteristic of brackish marshes and the banks of tropical rivers in company with Mangrove plants and the “Stemless Palm” Nipa. This species exhibits the anatomical characters of a water-plant and affords an interesting parallel with some Palaeozoic ferns (species of Psaronius) which probably grew under similar conditions.
The Anatomy of Ferns.
The text-book accounts of fern-anatomy convey a very inadequate idea of the architectural characters displayed by the vascular systems of recent genera. When we are concerned with the study of extinct plants it is essential to be familiar not only with the commoner recent types, but particularly with exceptional or aberrant types. The vascular system of many ferns consists of strands of xylem composed of scalariform tracheae associated with a larger or smaller amount of parenchyma, surrounded either wholly or in part (that is concentric or bicollateral) by phloem: beyond this is a pericycle, one layer or frequently several layers in breadth, limited externally by an endodermis, which can usually be readily recognised. The vascular strands are embedded in the ground-tissue of the stem consisting of thin-walled parenchyma and, in most ferns, a considerable quantity of hard and lignified mechanical tissue. The narrow protoxylem elements are usually characterised by a spiral form of thickening, but in slow-growing stems the first-formed elements are frequently of the scalariform type.
A study of the anatomy of recent ferns both in the adult state and in successive stages of development from the embryo has on the whole revealed “a striking parallelism[730]” between vascular and sporangial characters in leptosporangiate ferns. For a masterly treatment of our knowledge of fern anatomy from a phylogenetic point of view reference should be made to Mr Tansley’s recently published lectures: within the limits of this volume all that is possible is a brief outline of the main types of vascular structure illustrated by recent genera.
Fig. 237.
- Matonia pectinata (petiole).
- M. pectinata (stem).
- Gleichenia dicarpa (stem): p, petiole; pp, protophloem; position of protoxylem indicated by black dots.
- Matonidium.
- Trichomanes reniforme: pp, protophloem.