Systematics, Reproduction and Phylogeny of Isoetes

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Isoetes:-

1. Systematics:-

Division:- Lycophyta

Class:- Ligulopsida

Order:- Isoetales

Family:- Isoetaceae 

Genus:- Isoetes 

2. Habit and Habitat:-

> Isoetes includes 60 to 70 species and is world wide in distribution. Nearly 10 species are reported

from India, such as:-

i. Isoetes coromandelina:- Reported from Kerala, Tamil nadu, Andhra Pradesh Maharashtra and Gujrat Padampur and Bengal Madhya Pradesh, Uttar Pradesh, Bihar and Orissa.

ii. Isoetes coromandelina Sub Sps. branchyglossa:- West Bengal.

iii. Isoetes bilaspurensis:- Madhya Pradesh.

iv. Isoetes dixitii:- Maharashtra.

v. Isoetes indica:- Uttar Pradesh, Madhya Pradesh and Tamilnadu.

vi. Isoetes panchananii:- Madhya Pradesh and Maharashtra.

vii. Isoletes pantii:- Madhya Pradesh.
viii. Isoetes sahyadriensis:- Maharashtra and Tamil nadu.
ix. Isoetes Sampath Kumarnii:- Maharashtra and Tamil nadu.
x. Isoetes unilocularis
3. Study of sporophyte:-
> Sporophyte plant body are small ranging from 5 cm to 60 cm. in length, herbaceous and perennial. 
> It consist of short, fleshy, upright corm-like axis - which is very specialized. 
> On the upper surface of the axis are born densly-crowded cluster of quill-like leaves, hence called quillwort. 
> On the basal part of the axis there are two or three deep vertical grooves that makes it two or three lobed. (In Isoetes coromandelina is four or five-lobed). 
> The roots are borne on the side of the grooves. 
> The upper leaf bearing parts is known as stem and lower parts contain roots, which is designated as rhizomorph. 
> The vertical growth is very slow. 
> In width it range from 0.5 to 2.5 cm. but in some forms it may reach upto 4 cm or more.
> Roots in Isoetes arise from the lower portion of the fleshy axis and are unbranched rarely branched dichotomously.
> Leaves are borne on the upper surface of the axis. 
> Each leaf has a thickened and expanded colourless base and a tapering distal portion which is owl shaped and pointed. 
> The leaves are 30 to 70 cm long and are arranged in a dense spiral with phyllotaxy of 3/8, 5/13 and 8/21. 
> A persistent more or less triangular ligule are found on the concavity. 
> The leaf has a median unbranched vein.
> Apical growth is initiated by a group of apical cells. 
> Internal structure of corm:- 
- Longitudinal section of the Corm in two planes gives detailed idea of anatomical feature.
i. In a plane at right angles to the basal groove and through each lobe.
ii. In the plane along the externally seen basal groove.
- In the centre of the corm is stele. Stele is protostele with superparenchymatized layer of pith. 
- The secondary growth is by cambium which is composed of two parts:
i. A lateral meristem which surrounds the upper portion of the vascular cylinder and encloses the sides of the lower root bearing portion of the corm.
ii. A basal meristem which is continuous with the lateral meristem arranged to form a ribbon.
> Internal structure of Leaf:- 
- Cross section reveals semicircular or more or less rhombic. 
- Mesophyll cells undifferentiated. 
- The stomata are situated over the air cavities. 
- The vascular bundle is small and collateral.
- The ligule remain situated on the adaxial side of the leaf base consist of only of Parenchyma. 
- Its base thickens and penetrate or little into the leaf as glossopodium. 
4. Reproduction:-
> Every leaf in both genera is potentially a sporophyll because each bears sporangium, normal or abortive. 
> The sporophyte are heterosporous. 
> The development of sporangium is of eusporangiate type. 
> The mature sporangia are of two kinds: microsporangia and megasporangia but they are much alike in size and appearance. 
> The microsporangia bear numerous (1,50,000 to 2,50,000) small Microspore. 
> Megasporangia bear 50 to 500 large megaspore. 
> Spore germinates as soon as they are shed from the microsporangium.
> The mature microgametophyte is wholly endoscopic and consists of a small prothallial cell and one antheridium. 
> Antherozoids are biflagellated. 
> The ninecelled microgametophyte is more reduced than in any other pteridophyte.
> The megaspore consist of considerable amount of reserve food material and large nucleus. 
> The archegonia are developed from the superficial cell. 
> At maturnity, the neck canal cell and ventral canal cell disintegrate and the female gametophyte is initially endospore. 
> No suspensor is formed and the embryogeny is exoscopic.

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.

> Fossilised specimens of I. beestonii have been found in rocks dating to the latest Permian.
> Quillworts are considered to be the closest extant relatives of the fossil tree Lepidodendron, with which they share some unusual features including the development of both wood and bark, a modified shoot system acting as roots, bipolar growth, and an upright stance.
> Studies indicates that the Isoetes crown group which exist today evolved from a single lineage in the early Cenozoic.