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In this essay we will discuss about archaeopteris which belongs to the family of progymnospermopsida. After reading this essay you will learn about:- 1. Habit of Archaeopteris 2. Morphology of Archaeopteris 3. Stem Anatomy 4. Reproductive Organs.
Essay # 1. Habit of Archaeopteris:
Large trees up to 18 m high with a large crown of spirally arranged pseudomonopodial branches (Fig. 1.91) in which the penultimate branches bear both ultimate branches and leaves in a spiral.
Essay # 2. Morphology of Archaeopteris:
Both the branches and leaves show determinate growth which are developed in the same organotactic spiral. There is a continuity between the xylem of the branches and the parental axes. This suggests that the branches of Archaeopteris were not produced by lateral buds, rather developed from primordia like those that produced the leaves.
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The fertile ultimate branches may occur as the most basal units of a branch system. In some species, both fertile branches (strobili) and sterile branches (leaves) are intermixed and are arranged spirally (Fig.1.92). The fertile branches bear one or two rows of fusiform sporangia on adaxial surface.
Archaeopteris shows considerable variations of ultimate branches in different species, suggesting the origin of megaphyllous leaves from a planated and webbed telome truss (Fig. 1.93A-D). In A fissilis (Fig. 1.93A), the leaves are represented by planated terete telomes that dichotomise two to three times.
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In A. macilenta (Fig. 1.93B), the telomes are planated with various degrees of incomplete webbing so that the distal margins of leaves are deeply incised. The wedge-shaped megaphyllous leaves of A. halliana (Fig. 1.93C) and A. obtusa (Fig. 1.93D) are formed due to the complete webbing where a single vein enters the decurrent base and forms repeated dichotomies.
Essay # 3. Stem Anatomy of Archaeopteris:
Several logs of Archaeopteris (Callixylon) were discovered from the Upper Devonian strata which were up to 10 m long and about 1.5 m in diameter. Archaeopteris possesses a eustele with a ring of mesarch primary bundles surrounding a pith (Fig. 1.94A). The primary bundles are surrounded by thick-walled, compactly arranged secondary xylem (pycnoxylic wood), composed of radially arranged tracheids and narrow rays.
In a few specimens, the wood shows growth rings. The wood rays are generally narrow, but may be broader in the region of the pith. The rays are variable in height, comprising of ray tracheids and ray parenchyma. The secondary tracheids have circular bordered pits arranged on the radial walls in groups (6-20) and are multiseriate in most species.
These groups of pits are arranged horizontally and are separated by unpitted tracheid walls (Fig. 1.94B). The crassulae are often present in the tracheids and those are formed as a result of the separation of wall layers in the tracheids.
Essay # 4. Reproductive Organs of Archaeopteris:
Each fertile leaf of Archaeopteris (e.g., A. halliana) is comprised of a terete, dichotomously branched telome truss bearing one or two rows of fusiform sporangia on adaxial surface (Fig. 1.95A).
Although homosporous, heterospory has been demonstrated in several species of Archaeopteris. Arnold (1939), for the first time, reported heterospory in Archaeopteris halliana. In heterosporous species (A. halliana, A macienta, A fissilis), the micro- and megasporangia are more or less alike in terms of their sizes.
The sporangia are thick-walled with longitudinal dehiscence. Sometimes, stomata are present on the epidermal cells of the sporangia. A single microsporangium contains more than 100 microspores of 33-70 pm in diameter (Fig. 1.95B), while a single megasporangium contains 16-32 megaspores of 110-500 pm in diameter (Fig. 1.95C).
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The microspore has a long trilete laesura. The proximal surface of the microspore is psilate, while the distal surface is ornamented. The microspores of Archaeopteris are similar to the dispersed form genus Cyclogranisporites.
The megaspores are also trilete with elevated laesura scar. Like microspores, the proximal surface of megaspore is psilate, while the distal surface is ornamented with folds. The megaspores of Archaeopteris are included in the dispersed form genus Biharisporites.
