Systematics, Reproduction and Phylogeny of Sphenophyllum

Sphenophyllum:-

1. Systematics:-

Division:- Sphenophyta

Class:- Sphenopsida

Order:- Sphenophyllales

Family:- Sphenophyllaceae 

Genus:- Sphenophyllum 

2. Habit and Habitat:-

> Sphenophyllum is found in rocks ranging in age from Late Devonian to late Permian and now it is a fossil.

> It was shruby and climber in nature like vine.

> These plants first appear in upper Devonian period and flourished until the lower Triassic. 

> These are found in the Barakar Coalfield and coalfields in Bihar, Bengal, Satpura basin and Sikkim. 

> It is the compression- impression fossil.

> Important species:-

i. S. speciosum

ii. S. coneifolium

3. Study of sporophyte:-

>  The genus is characterized by delicate whorls of leaves arising from nodes on thin, woody axes of various orders, and was a scrambling plant that formed a ground cover of dense, multi-branching thickets on both clastic and peat-forming substrates 

> Leaves are heterophyllous, where morphology of the leaves depends on the order of the branch to which they were originally attached. 

> Whole-plant reconstructions do not yet exist for Sphenophyllum, in part because rooting organs are rarely found in connection with aerial parts of the plant.

> Root:- The roots are adventitious borne along the internodes, rarely produced on the lateral branch. 

> Stem:-

- The stems are jointed with non-alternating ribs. 

- The stem is delicate in spite of secondary thickening and thought to be herbaceous climbing stem. 

- The diameter of the stem seldom exceed 1 cm in diameter. 

- It was irregularly branched (dichotomous), which arise from nodes in position alternating with the leaves. 

- Stem having root like anatomy. Actinostele of only triangular nature.

> Leaf:- 

- A whorl of 6-18 leaves were present on each node. 

- The leaves are constricted at the base and expanded distally. 

- The leaves are supplied with single vein, which divided into two on lamina. 

- The leaves are variously shaped.

i. Obovate leaf:- In S. emerginatum and S. majus. 

ii. Furcated leaf (Cuniate leaf):- S. myriophyllum. 

Note:- S. cuneifolium bear both types of leaves. 

> Internal structure:-

i. Epidermis:- Periderm observed in fossil records. 

ii. Cortex:- Well preserved massive sclerenchymatous. 

iii. Stele:- 

- Protostele-triarch.

- Xylem is entirely made up of tracheid, having multiseriated bordered pit on the lateral walls. 

- Anatomically the structure of leaf shows mesomorphic to xeromorphic features, although these plants occurred within coal swamps. 

- The chlorenchyma was reduced and the dermal and hypodermal tissues were sclerenchymatous.

> Secondary Growth:- 

- Took place from a uni- or bifacial cambium, similar to that of carboniferous Lycopsids.

- Primary phloem & secondary phloem consisted of thin-walled sieve elements. 

- The sieve cells were axially elongated with tapered end walls. 

- The sieve elements were of relatively small diameter in the areas opposite the protoxylem arms.

4. Reproduction:-

> Reproductive organs of the Sphenophyllales consist of aggregations of sporangiophores and bracts that form cones or strobili. 

> Fructification:- Terminal cones, describe under the genus Bowmanites. There are various species formed based upon cone size:

i. S. trifurcatus (6cm X 1cm) = Homosporous

ii. S. speciosum (12cm X 2cm) = Homosporous

iii. S. dawsonii (30cm X 2cm) = Homosporous

iv. S. delectus (Length is not determined) = Heterosporous (Rare)

> The axis, which in structure resembles the vegetative stem in its primary condition, bears numerous verticals of bracts, those of each verticil being coherent in their lower part, so as to form a disc or cup, from the margin of which the free limbs of the bracts arise. 

> 1 or 2 sporangiophores develop from one bract.

> The sporangia, which are about twice as numerous as the bracts.

> 1 or 2 sporangia were hanging down, it means pendulous type.

> The sporangium is attached to the enlarged distal end of its pedicel, from which it hangs down, so as to suggest an anatropous ovule on its funiculus. 

> Dehiscence appears to have taken place at the free end of the sporangium; the spores are numerous, and, so far as observed, of one kind only. 

>Each sporangiophore is traversed throughout its length by a vascular bundle connected with that which supplies the subtending bract.

5. Phylogeny:- 

> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.

> The probable relationships within Equisetidae are shown in the cladogram below. 

> The possible position of Ibyka has been added.