Origin of Secondary Wood, Primary Vasculature and Leaf in Gymnosperms

In this article we will discuss about the Origin of Secondary Wood, Primary Vasculature and Leaf in Gymnosperms.

Origin of Secondary Wood in Gymnosperms:

Under-mentioned are some of the studies which indicate that the secondary wood of the modern gymnosperms might have originated in the Progymnospermopsida of Upper Devonian period:

(1) The frond-genus Archaeopteris of Progymnospermopsida had very large fern-like fronds which bore numerous stalked sporangia of two kinds, some larger and other smaller. The stem- genus Callixylon, which was reported by Beck (1960) in close organic connection with Archaeopteris, had coniferous wood.

Due to the combination of the pteridophytic and gymnospermous characters, this discovery of Beck throws some light on the gymnospermous ancestry in at least regarding the origin of the secondary wood.

(2) The secondary wood of Tetraxylopteris, another progymnosperm, also had several gymnospermous characters. Presence of secondary wood, round bordered pits of tracheids and sclerotic outer cortex in Tetraxylopteris are some of the characters of its pteridospermous affinities. Another interesting gymnospermous character of this genus is the uniform pitting of its radial and tangential walls.

Origin of Primary Vasculature or Evolution of Stele in Gymnosperms:

Majority of the modern gymnosperms possess eustele made up of distinct strands.

Most of the botanists believe that eustele has originated from the primitive protostele, and following two views are prevalent to prove this i.e. Jeffrey’s view and Geyler’s view:

1. Jeffrey’s View:

This view was proposed by Jeffrey (1902, 1917) and supported by several botanists including Eames, Smith and Zimmermann. According to this view, the first step in the evolution of eustele from primitive protostele was the appearance of centrally located pith.

The condition, thus developed, was siphonostelic. Jeffrey believed in the extrastelar origin of the pith. He opined that this pith appeared by the enclosure of some tissue of the cortex within the stele, and simultaneously the leaf gaps appeared.

On the other hand, some workers believe that some parenchyma appeared in the central xylem core and thus the pith was of intrastelar origin. In the next step the overlapping leaf gaps dissected the so-formed siphonostele and this resulted in the formation of separate bundles or meristeles. Thus formed meristeles form a dictyostele.

In the last stage of the evolution, there was a loss of structures such as internal endodermis, internal pericycle and internal pith of amphichroic siphonostele, and the ultimate result was the formation of separate or discrete collateral vascular bundles. Thus resulted stele was eustele, a characteristic of gymnosperms.

2. Geyler’s View:

This view was proposed by Geyler (1867) and supported by Scott (1923), Beck (1960, 1962) and Namboodiri and Beck (1968). According to this view the origin of eustele from protostele through siphonostele in gymnosperms, as proposed by Jeffrey, cannot be explained because there are no leaf gaps in the gymnospermous eustele.

They proposed that gymnospermous eustele has evolved through longitudinal dissection of the protostele.

Following might have been the successive stages of the evolution of gymnospermous eustele by the longitudinal dissection of protostelic condition according to Namboodiri and Beck (1968) and Beck (1970):

i. Some members of Progymnospermopsida, such as Aneurophyton and Tristichia, had a massive three-ribbed primitive protostele. From the outer surface of such a stelar column, the leaf traces diverged radially to the appendages as shown in Fig. 23.1 A.

ii. During the course of evolution, the next stage is seen in some Upper Devonian genera, such as Archaeopteris, Calamopitys, Eristophyton and Stenomyelon. A tendency of gradual dissection of the protostele into three or more columns is observed in these genera.

The pith appeared consequently during this process of evolution. From the outer margins of the vascular strand, the traces diverged by tangential division, and there was no formation of the leaf gaps (Fig. 23.1 B).

iii. The next stage is also observed in some species of Archaeopteris and Calamopitys (Fig. 23.1C) in which the stele had larger pith and separate vascular strands near the periphery. The stelar bundle bifurcated at the time of leaf trace formation and formed an outer and an inner bundle. The outer bundle diverged as a leaf trace and the inner bundle continued as the reparatory strand with no leaf gap at all.

iv. The next step (Fig. 23.1 D) in the evolution of gymnospermous eustele is observed in genera such as Callistophyton, Lyginopteris, Mesoxylon, Pitys, Poroxylon and several modern Coniferales. In this stage, the nature of the division of the sympodium changed from a division forming radially arranged bundles to a division of the sympodium resulting in two bundles which were placed tangentially at the initial level.

v. The final stage is observed in the primary vasculature of several living Coniferales. In this stage, in addition to the initial tangential orientation of the leaf trace and reparatory trace (sympodial segment), the reparatory trace takes an undulating upward path as shown in Fig. 23.1 E.

vi. The vascular pattern of the advanced members such as Ginkgo, Ephedra, etc. developed due to an increase in the number of traces supplying the leaf from one to two and also due to the change in the direction of divergence of traces.

Beck (1967) opined that eustelic organisation of living gymnosperms developed due to two lines of evolution, and both these lines diverged from Aneurophytales of Progymnospermopsida. One of these lines diverged in the direction of Cycadophytic gymnosperms through Calamopitys and the other line in the direction of coniferophytic gymnosperms by way of Archaeopteris.

Origin of Leaf in Gymnosperms:

While Zimmermann (1930) explained the origin of microphyllous and megaphyllous leaves among vascular plants through his widely accepted “Telome Theory”, Bower (1935) discussed this in his well-known “Enation Theory”.

Beck (1970) explained that origin and evolution of gymnospermous leaves followed the following steps:

1. A three-dimensional and dichotomously branched ultimate branching system was present in some Middle and Upper Devonian Progymnospermous plants, such as Aneurophyton and Tetraxylopteris.

2. The ultimate appendages evolved into leaf-like structures in some plants of Middle Devonian period, such as Actinoxylon.

3. Simple, wedge-shaped and laminated leaves appeared first in some species of Archaeopteris.

4. These evolved into leaves having slender dichotomous divisions as in Archaeopteris fissilis.

5. In members of Progymnospermopsida, the three-dimensional ultimate branch system passed through the processes of planation and lamination and evolved into the simple leaves.

6. Flattened lateral branches were present in some Progymnosperms such as Siderella and some species of Archaeopteris. These branches represented an intermediate stage in the evolution of compound leaves of modern gymnosperms.

7. The cauline characteristics (such as bifurcation of the main axis and presence of radially symmetrical vascular system) of the fronds of some Lower Carboniferous Pteridospermales (e.g. Tristichia and Tetrastichia) indicate about the origin of compound leaf of modern gymnosperms from the lateral branch system.

8. Dissection of the lamina of large simple leaves might have also originated into the compound leaves of living gymnosperms.