Equisetum

The family Equisetaceae includes a single living genus Equisetum, which is the only living representative of the entire race, that has become extinct. Commonly known as the Horsetail plant. 

1.0Habitat 

The Equisetum is a versatile plant that can thrive in various habitats but especially does well in damp conditions. For example, E. debile can grow in wet, moderate, and dry environments, showcasing the plant's adaptability. This adaptability has allowed the genus to endure over time. Typically, these plants are found along riverbanks, streams, and swampy areas. Silica is a common component in the cell wall of these plants, giving their stems rough texture.

2.0Classification of Equisetum

3.0Structure of Equisetum 

Equisetum's plant body, which represents the sporophyte generation, is differentiated into stems, roots, and leaves. 

Stem 

The main stem of the Equisetum is an extensively branched, dark brown rhizome that produces branches from its nodes. This rhizome penetrates deeply into the soil, giving rise to both aerial and subterranean branches. Aerial branches are monopodial, with distinct nodes and internodes. At the nodes, whorls of leaves are laterally fused at the base. While some shoots are highly branched, green, and purely vegetative, others are unbranched, brownish, and fertile, culminating in a terminal strobili for spore production. 

Root

The roots of the Equisetum plant typically emerge from the rhizome. Interestingly, except for the initial root of the embryo, all subsequent roots are adventitious and develop endogenously. These roots originate from the bases of the branch primordia at each node, resulting in a whorled arrangement. The roots are highly branched and feature a protective root cap at their tips. This distinctive root system contributes to the plant's stability and nutrient absorption.

Leaves

Equisetum has small, simple leaves that are uniformly sized (isophyllous) and arise in circular groups at each node. The number of leaves can differ between species or within the same species. At each node, the leaves are fused into a protective sheath around the base of the stem segment (internode). The sheath has free and pointed tips resembling teeth. These leaves primarily serve a protective role and do not contribute to photosynthesis.

4.0Diagram of Equisetum 

5.0Reproduction in Equisetum

Equisetum reproduces by both vegetative and sexual reproduction (spores). 

Vegetative Reproduction

In some species, the primordia on the rhizome branch remain short and develop into round or oval tubers. Each tuber grows into a new plant when detached from the parental rhizome. The cells of the tuber are filled with starch, and every tuber has an outer thick-walled protective layer of cells. 

Reproduction by Spores 

Equisetum is homosporous, meaning it produces a single type of spore. These spores develop within structures called sporangia, which are carried by sporangiophores. These sporangiophores are grouped together into a condensed structure known as a strobilus or cone.

Strobilus

In Equisetum strobilus or cone is concerned with the production and release of spores. Strobili are consistently terminal and singular. In many species, there is no distinction between sterile and fertile shoots, with each main aerial shoot carrying a terminal strobilus. These aerial shoots serve both photosynthesis and reproduction. In contrast, some species exhibit a separation between fertile and sterile aerial shoots.

The cone of Equisetum is distinctive, consisting of a central thick axis bearing densely crowded T-shaped peltate scales called sporangiophores. These sporangiophores are arranged in successive whorls, alternating somewhat irregularly. The number of sporangiophores in each whorl varies.

Sporangium 

A mature sporangium in Equisetum is a long sac filled with numerous small, equal-sized haploid spores. These spores are uniform in size (homosporous). The mature sporangium has a single layer of cells forming its wall, with spirally thickened cell walls. In its early stage, the sporangium has 2-4 layers of thick walls. The innermost layer, called the tapetum, originates from the outermost sporogenous cells. As spores develop, the inner layers and tapetum become disorganized.

As the strobilus matures in Equisetum, the axis elongates slightly, causing the sporangiophores to loosen and separate. Subsequently, as water is lost, the sporangiophores shrink and break apart, exposing the sporangia.

Spores 

In Equisetum, young spores are initially green and covered by a thin cellulose wall. As they mature, they become larger, rounded, and filled with numerous chloroplasts. The mature spores have a thick, four-layered wall: the outermost epispore, middle perispore, third layer (exospore), and innermost endospore or endosporium.

The epispore splits along spiral lines into two long bands called elaters, which coil tightly around the spores until maturity. Elaters are hygroscopic, remaining coiled in moist air. When dry, they stretch out crosswise, appearing as four distinct appendages. Notably, Equisetum elaters differ from those of bryophytes, and they are haploid in nature.

Germination of Spore

Equisetum follows homosporous reproduction. Haploid spores initiate the gametophytic generation by germinating. These green spores are viable for a brief period. Upon absorbing water, the spore swells, and its middle layer ruptures. This results in the formation of two cells: a small, lenticular primary rhizoidal cell and a larger prothallial or gametophytic cell. The rhizoid cell has few or no chloroplasts, while the prothallial cell contains many chloroplasts and oil droplets.

The prothallial cell undergoes division in different planes, giving rise to the rest of the prothallus. This prothallus bears sex organs and reproduces through a sexual method.

Archegonium

The archegonium in Equisetum starts as a surface cell from the outer edge of the meristem. It divides into two cells: the outer primary cover cell and the inner central cell. The primary cover cell divides into four neck cells, forming the archegonial neck that extends vertically above the thallus. The central cell divides into a primary canal cell and a primary ventral cell.

The primary ventral cell further divides into a ventral canal cell and an egg cell. At maturity, the archegonium consists of a projecting neck with three to four tiers of neck cells in four rows, two neck canal cells of different sizes, a ventral canal cell, and an egg at the base. The final stage involves the gelatinization of the neck canal cells and the ventral canal cell, creating a passage for sperm entry during fertilization.

Antheridium 

Antheridia in Equisetum develop later than archegonia, typically when the prothallus is several months old. There are two types of antheridial development: embedded and projecting. The embedded type forms on the lower, cushioned part of the prothallus, while the projecting type occurs in starved prothalli at the apices or margins of erect lobes.

The antheridial initial divides through periclinal division, resulting in outer primary jacket and inner primary androgonial cells. The jacket initial then undergoes anticlinal division to create a single-layered jacket wall for the antheridium.

Repeated divisions of androgonial cells generate numerous cells that, upon metamorphosis, give rise to spermatids or antherozoids. These antherozoids are released through a pore formed by the separation of the apical jacket cell.

Fertilization

Water is crucial for fertilization in Equisetum. Spermatozoids are drawn towards the scent of malic acid found in the mucilage surrounding the exposed archegonial necks. Several spermatozoids enter the neck and reach the venter, but only one successfully achieves fertilization. This process transforms the haploid egg into a diploid structure known as the zygote or oospore. The oospore divides by a transverse wall into an upper epibasal cell and a lower hypobasal cell. The oospore is the pioneer structure of the sporophyte. 

6.0Life cycle of Equisetum 

7.0Common species of Equisetum

Equisetum hyemale : Equisetum hyemale plant is also known as rough horsetail

Equisetum arvense : also known as field horsetail.