In this article we will discuss about the morphology of equisetum with the help of a suitable diagram.
Equisetum, with some 15 or more species, is distributed all over the world except in Australia and New Zealand. It is commonly known as ‘horse-tail’ or ‘scouring rushes’. The most conspicuous features of the plant are jointed nature of stems (Fig. 9.1 A), intercalary growth, whorls of branches, small leaves around nodes, and sandpaper-like texture.
Due to this texture Equisetum has been nicknamed as ‘scouring rushes’ and its whorls of branches make it look like the tail of a horse. The rough texture of plant is due to deposition of silica in cells on outer surface. Silica helps to protect the plant against predators and pathogens and prevents excessive water loss.
The plant with an interesting range of growth habits grows in a variety of habitats, ranging from wet places (ponds and marshes) to damp shaded places, along river banks, to exposed xeric conditions. E. arvense is the most common species.
In some tropical localities, Equisetum is a weed and, because of its poisonous contents, is of concern to livestock owners. Horses are especially susceptible to its toxins. Some species accumulate minerals including gold (up to four and half ounce per ton). This feature of the plant is valuable in prospecting for new ore deposits.
All species have a perennial, subterranean, much branched, rhizome (Fig. 9.1 A) that helps the plant to spread prolifically. From the rhizome arise aerial axes, which are sparsely to highly branched. The aerial axes in E. hyemale are sparsely branched and develop dwarf branches in second or third year whereas they are highly branched in E. sylvaticum and E. arvense, showing branching of third or fourth order in the same year.
In most species the aerial axes are of limited size (10-60 cm tall) but exceptionally E. giganteum, as the name suggests, may attain a height of 10-13 metres.
Some forms are evergreen; in others the aerial portions die out every year. Aerial stems in most species (Fig. 9.1 A, B) are of two types, fertile and sterile. The former are colourless and unbranched with a strobilus at the apex. These appear before the sterile vegetative green axes with whorls of lateral branches.
Exceptionally, in some species the fertile branches are green and may be branched or unbranched. In E. pratense the fertile shoot is green and after dispersal of spores gives rise to vegetative branches lower down. Also in E. palustre the fertile shoot is green and branched and every lateral branch ends in a cone. In species lacking heteromorphic branches, viz., E. myriochaetum and E. debile, the cones are borne on normal green aerial branches.
The stems (rhizome and aerial axes) are ridged and differentiated into nodes and internodes. Around each node are small scale leaves (Fig. 9.1 A, B). The leaves are joined at their bases, forming a sheath around the node, and their free distal ends give the appearance of a frill. The leaves are normally of the same number as the ridges on stem. Each ridge corresponds to a leaf in the node above.
Ridges in successive internodes alternate with one another and so also the leaves in successive leaf sheaths. As a departure from this regularity, the number of leaves diminishes in successive sheaths in distal region of the stem. It can be seen in a bud (Fig. 9.1C, D).
In Fig. 9.1C, an outline diagram of cross section of bud, are seen two whorls; the outer whorl has seven leaves and inner one has five. Two leaves which are non-alternating are massive, each with double leaf traces. Similarly, in Fig. 9.1D, there are three whorls of eight, seven and five leaves, and non-alternating leaves are anomalous in size within their whorls.
On this basis, an interesting generalization has been derived. The non-alternating leaves, which are anomalous in size, frequently have double leaf traces that vary in degree of doubleness. Another interesting feature of leaf is the presence of hydathodes along the mid vein.
Alternating with each leaf at the node is a branch meristem, or a branch primordium. From these meristems, on aerial axes arise branches but they remain inactive on rhizomes. Rarely, the meristems on rhizome are active and form short round branches (tubers) that are only one internode long (Fig. 9.1 A) and on separation secure vegetative propagation. In some species on aerial axes also, these meristems are inhibited from growing. It is possibly due to apical dominance.
In E. hyemale which is sparsely branched, the branch meristems can be made to develop if the stem is decapitated. Each branch meristem has a conspicuous stem apical cell and primordia for root and leaf. The root primordium is inhibited from growing on aerial axes but forms root on rhizome.
Normally, one root develops from one meristem but in E. sylvaticum several roots develop from each meristem. The roots at the rhizome nodes and at the base of aerial axes are, therefore, adventitious. The embryonic root is short-lived.