PAST HISTORIES OF PLANT FAMILIES
VIII. The Horsetails
The horsetails of to-day all belong to the one genus, Equisetum, among the different species of which there is a remarkably close similarity. Most of the species love swampy land, and even grow standing up through water; but some live on the dry clay of ploughed fields. Wherever they grow they usually congregate in large numbers, and form little groves together. They are easily recognized by their delicate stems, branching in bottle-brush fashion, and the small leaves arranged round them in whorls, with their narrow teeth joined to a ring at the base. At the end of some of the branches come the cones, with compactly arranged and simple sporophylls all of one kind. In England most plants of this family are but a few inches or a foot in height, though one species sometimes reaches 6 ft., while in South America there are groves of delicate-stemmed plants 20 ft. high.
The ribbed stems and the whorls of small, finely toothed leaves are the most important external characteristics of the plants, while in their internal anatomy the hollow stems have very little wood, which is arranged in a series of small bundles, each associated with a hollow canal in the ground tissue.
The family stands apart from all others, and even between it and the group of Lycopods there seems to be a big gap across which stretch no bonds of affinity. Has the group always been in a similar position, and stood isolated in a backwater of the stream of plant life?
In the late Tertiary period they seem to have held much the same position as they do now, and we learn nothing new of them from rocks of that age. When, however, we come to the Mesozoic, the members of the family are of greater size, though they appear (to judge from their external appearance) to have been practically identical with those now living in all their arrangements. In some beds their impressions are very numerous, but unfortunately most are without any indication of internal structure. Fossils from the Mesozoic are called Equisetites, a name which indicates that they come very close to the living ones in their characters. In the Lower Mesozoic some of these stems seem to have reached the great size of a couple of feet in circumference, but to have no essential difference from the others of the group.
When, however, we come to the Palæozoic rocks we find many specimens with their structure preserved, and we are at once in a very different position as regards the family.
First in the Permian we meet with the important genus of plant called Calamites, which were very abundant in the Coal Measures. Many of the Calamites were of great size, for specimens with large trunks have been found 30 ft. and more long, which when growing must certainly have been much taller than that. The number of individuals must also have been very great, for casts and impressions of the genus are among the commonest fossils. They were, in fact, one of the dominant groups of the period. Like the Lycopods, the Equisetaceæ reached their high-water mark of development in the Carboniferous period; at that time the plants were most numerous, and of the largest size and most complicated structure that they ever attained.
As will be immediately suspected from analogy with the Lycopods, they differed from the modern members of the family in their strongly developed anatomy, and in the strength and quantity of their secondary wood. Yet in their external appearance they probably resembled the living genus in all essentials, and the groves of the larger ones of to-day growing in the marshes probably have the appearance that the palæozoic plants would have had if looked at through a reversed opera glass.
Fig. 104 is a photograph of some of the small branches of a Calamite, in which the ribbed stem can be seen, and on the small side twigs the fine, pointed leaves lying in whorls.
In most of the fossil specimens, however, particularly the larger ones, the ribs are not those of the true surface, but are those marked on the internal cast of the pith.
Among tissue petrifactions there are many Calamite stems of various stages of growth. In the very young ones there are only primary bundles, and these little stems are like those of a living Equisetum in their anatomy, and have a hollow pith and small vascular bundles with canals associated. The fossil forms, however, soon began to grow secondary wood, which developed in regular radial rows from a cambium behind the primary bundles and joined to a complete ring.
A stem in this stage of development is seen in fig. 105, where only the wood and internal tissues are preserved. The very characteristic canals associated with the primary bundles are clearly shown. The amount of secondary wood steadily increased as the stems grew (there appear to have been no “annual rings”) till there was a very large quantity of secondary tissue of regular texture, through which ran small medullary rays, so that the stems became increasingly like those of the higher plants as they grew older. It is the primary structure which is the important factor in considering their affinity, and that is essentially the same as in the other members of the family in which secondary thickening is not developed. As we have seen already in other groups of fossils, secondary wood appears to develop on similar lines whenever it is needed in any group, and therefore has but little value as an indication of systematic position. This important fact is one, however, which has only been realized as a result of the study of fossil plants.
The longitudinal section of the stems, when cut tangentially, is very characteristic, as the bundles run straight down to each node and there divide, the neighbouring halves joining so that the bundles of each node alternate with those of the ones above and below it.
The leaves which were attached at the nodes were naturally much larger than those of the present Equisetums, though they were similarly simple and undivided. Their anatomy is preserved in a number of cases, and was simple, with a single small strand of vascular tissue lying in the center. They had certain large cells, sometimes very black in the fossils, which may have been filled with mucilage.
The young roots of these plants have a very characteristic cortex, which consists of cells loosely built together in a lacelike fashion, with large air spaces, so that they are much like water plants in their appearance. Indeed, so unlike the old roots and the stems are they, that for long they were called by another name and supposed to be submerged stems, but their connection with Calamites is now quite certain. As their woody axis develops, the secondary tissue increases and pushes off the lacelike cortex, and the roots become very similar in their anatomy to the stems. Both have similar zones of secondary wood, but the roots do not have those primary canals which are so characteristic of the stems, and thereby they can be readily distinguished from them.
The fructifications of the Calamites were not unlike those of the living types of the family, though in some respects slightly more complex. Round each cone axis developed rings of sporophylls which alternated with sterile sheathing bracts. Each sporophyll was shaped like a small umbrella with four spokes, and stood at right angles to the axis, bearing a sporangium at each of the spokes. A diagram of this arrangement is seen in fig. 109.
A photograph of an actual section of such a cone, cut slightly obliquely through the length of the axis, is seen in fig. 110, where the upper groups of sporangia are cut tangentially, and show their grouping round the sporophyll to which they are attached.
A few single tetrads of spores are enlarged in fig. 111, where it will be seen that the large spores are of a similar size, but that the small ones of the tetrads are very irregular. They are aborting members of the tetrad, and appear to have been used as food by the other spores. In each sporangium large numbers of these tetrads develop and all the ripe spores seem to have been of one size.
In a species of Calamites (C. casheana), otherwise very similar to the common one we have been considering, there is a distinct difference in the sizes of the spores from different sporangia. The small ones, however, were only about one-third of the diameter of the large ones, so that the difference was very much less marked than it was between the small and large spores of the Lycopods.
Among the palæozoic members of the group are other genera closely allied to, but differing from Calamites in some particulars. One of these is Archæocalamites, which has a cone almost identical with that of the living Equisetums, as it has no sterile bracts mingled with the umbrella-like sporophylls. Other genera are more complex than those described for Calamites, and even in the simple coned Archæocalamites itself the leaves are finely branched and divided instead of being simple scales.
But no genus is so completely known as is Calamites, which will itself suffice as an illustration of the palæozoic Equisetaceæ. Though the genus, as was pointed out above, shows several important characters differing from those of Equisetum, and parallel to some extent to those of the palæozoic Lycopods, yet these features are more of a physiological nature than a systematic one, and they throw no light on the origin of the family or on its connection with the other Pteridophytes. It is in the extinct family dealt with in the next chapter that we find what some consider as a clue to the solution of these problems.