Keeping their organic evolution in view, the plant kingdom has been divided into four broad groups’ viz. Thallophyta, Bryophyta, Pteridophyta, and Spermatophyta. Generally the Thallophytes constitute the most primitive members while most modern plants are placed under the Spermatophyta group.
These are mostly microscopic, unicellular and most primitive plants without differentiation of root, stem, leaf etc. Algae, fungi, bacteria and diatoms are included in this group. These are rarely fossilised; though, their hard outer cover sometimes accumulate to form rocks.
The plants belonging to this group are mostly land plants. Most of them prefer marshy and swampy areas. Plants like liverworts and mosses belong to this group. The mosses develop leaves, stems, trunks and hair-like roots, though, the liverwort are considered primitive because of their thallus-like body without any differentiation. Rhynia is a rare example of a fossilised bryophyta.
These are fern-like plants with distinct roots, stems, branches and leaves. They suck water along with minerals from the soil by its roots and transport it up to their leaves where food is prepared in the presence of chlorophyll with the help of sunlight and carbon dioxide from air.
After the food is prepared, it is transported back to all parts of its body. For this purpose these plants specially develop vascular system. Though flowers, fruits and seeds are not developed, they generally propagate by means of ‘spores’. Four distinct types of pteridophytes have been recognised and they are ferns, equisetales, lycopodiales and sphenophyllales.
(i) Fern – These are plants with compound leaves and spores. In size, they range from small shrubs to large trees. These were wide spread during Mesozoic era and are preserved in Gondwana rocks.
(ii) Equisetales -The trunk of these plants are with nodes and internodes. Leaves are small and develop at the nodes while internodes are marked with vertical striations. They appeared during Mesozoic era and many are found preserved in Gondwana rocks.
(iii) Lycopodiales – These are creepers with branches and spirally arranged moss like leaves. The plants of this type originated during Devonian period and are preserved abundantly in the Gondwana rocks of Carboniferous age.
(iv) Sphenophyllales -These are herbaceous climbers. Stem jointed nodes and internodes, leaves arranged in whorls around then these plants became extinct at the end of the Palaeozoic era. Four of these types of plants are found in the rocks of Lower Gond period.
These are most modern plants with distinct root, trite branch, leaf, flower, fruit and seed. They propagate through seeds. S phylum Gymnosperms and Angiosperms are included in this phylum.
(A) Gymnosperms – Small evergreen pine like trees with unprotected naked seeds are included in this category. This sub-phylum includes six class’s viz. Cycadophylicales, Cycadales, CordaitalJ Ginkgoales, Coniferales and Gentales.
(i) Cycadophylicales – Though the plants of this group look like ferns, these are with true but naked seeds. The plants of this class form the major bulk of the Gondwana coal deposits. Glossopterh, Gangamopteris and Vertebraria are a few plant fossils found in the Lower Gondwana rocks.
(ii) Cycad ales – Plants with stout trunks marked by leaf scars. Ptilophyllum and Nilsonia are a few fossilised cycad ales preserved in Upper Gondwana rocks. These types of plants came to exist during the Permian period and are found to live even today.
(iii) Cordials – Tall trees with large trunk branching at the tip. Leaves with parallel venation and look like swords. This type of plants appeared during end of the Paleozoic era and became extinct before the advent of the Mesozoic era.
(iv) Ginkgo ales – Plants resembling conifers in appearance with fan like leaves range in age from Permian to Recent. Ginkgo ales with worldwide distribution are represented by only one species the Ginkgo.
(v) Conifer ales – Evergreen trees with needle-like leaves. They range in age from Permian to Recent. At present they are represented today by the pines.
(vi) Gentiles – These are small trees with a few living representative adopted to desert environment so far not found to occur as fossils.
(B) Angiosperms – These are most modern flowering plans propagating by seeds. The monocotyledon seed germinate with a single leaflet with parallel venation. They are characterized by cylindrical stem and fibrous roots.
The palm and grass type of trees are examples of the monocots. The dicotyledonous seeds, on the other hand, germinate with two leaflets, generally with reticulate venation. They range from small shrubs to very large evergreen trees of the rain forests.