The Protoleptosporangidae or the Osmundidae comprising of the single order Osmundales with the single family Osmundaceae is taken out of the Leptosporangiates as proposed by Hirmer (1927) as it is not truly leptosporangiate.
The Osmundales is a very ancient order represented by Thamnopteris and Zalesskya from late Permian and by Osmundites itself from Jurassic onwards.
The climax of the group was probably in the Jurassic after which they dwindled. Sahni has suggested its derivation from Grammatopteris (Lower Permian) of Botryopteridaceae after a detailed study of the anatomy. Several species of Cladophlebis fronds and petrified stems and roots of Osmundites sahnii are found in Rajmahal stage Upper Gondwana beds.
Osmundaceae is divided into three Genera:
Osmunda (14 spp. from temperate and tropical of both hemispheres), Todea (1 sp. from South Africa) and Leptopteris (61 spp. from Australasia and Melanasia).
The chromosome number of Osmundaceae is very constant in = 22) proving their primitiveness.
Osmunda claytoniana is known in the Himalayas from Kashmir to Khasia hills (1500 to 3000 metres). O. regalis is also known all over the Himalayas as well as in South India (1200 to 1900 metres).
The sporophyte of Osmunda has a short, upright or creeping, usually subterranean stem covered by persistent leaf bases and adventitious roots from the bases of the leaves. The other two genera have erect, 1 to 3 m high trunks. Stems of Osmunda may bifurcate once or twice. The apex of each stem bears a crown of pinnate or bi-pinnate leaves (Fig. 612A)which may be up to 3 metres long and show circinate ptyxis.
The leaves, when young, have a covering of pale-brown, uniseriate hairs some of which may be glandular and the leaves also develop stipule-like wings at base on which, again, there may be glandular hairs. Sonic species bear two kinds of leaves, sterile and fertile, while in other species fertile pinnae develop at the middle or at the end of sterile pinnae (Fig. 612A & B).
In Todea and Iisptopteris all leaves are of one kind(Fig. 612D). The leaves of Osmunda and Todea are as in other ferns but those of Leptopteris are filmy and only one layer thick as in Hymenophyllum.
The first developed portion of a stem may be protostelic. Later developed portions are dictyostelic with numerous leaf gaps (Fig. 613). Outer portion of cortex is dark-brown with highly sclerified cells and covered by numerous persistent sclerenchymatous leaf bases. Inner cortex is colourless and parenchymatous.
There is a distinct endodermis which is continuous in the numerous spirally arranged leaf traces. The leaf traces are C-shaped. There is a 2- to 4-layered pericycle. The phloem is outside the xylem patches. The xylem patches are oval to U-shaped, mesarch to endarch. Metaxylem is composed of large scalariform-pitted tracheides.
Pith is usually homogeneous but may have sclerenchyma in some species. Roots are endogenous.
The sporangia are marginal or terminal on stalks in Osmunda and superficial on the lower surface is the other two genera. They do not form sori and there is no coveting (Fig. 612B & D). The sporangia (Fig. 612C) are large, pyriform and with stout, short stalks. In the development of the sporangium there conspicuous initial cell which develops the sporangium capsule with a single layered jacket as in the Leptosporangiates.
But the massive stalk and. in some cases even a part of the capsule wall is formed by cells outside the initial so that the mode of development is intermediate between eusporangiate and leptosporangiate. A group of cells on one side of the jacket thickens at a late stage and forms a shield-shaped annulus.
A vertical zone of dehiscence is also formed on the top by some elongate cells. All the sporangia in a pinna mature together.
Spores are formed in large numbers, contain chloroplasts when ripe and germinate rapidly. The gametophyte is more or less heart-shaped, dark-green, multi-layered and with a thick midrib. It, thus, resembles Marattia and not the leptosporangiate ferns. Adventitious lobes are cut off from the prothallus and reproduced vegetatively.
The gametophyte is monoecious but the antheridia develop, first. The antheridia develop marginally, terminally or on the ventral surface and the development remind the eusporangiate type though the mature antheridia project above the general surface. The sperms are produced in large numbers and are coiled with numerous cilia at tips.
Their escape is by breaking the apical opercular cell. The archegonia develop on the ventral midrib or along its edges and have long necks about 8 cells high which project’ out of the prothallus.
The first division of the zygote is vertical as in the leptosporangiates but the differentiation of the embryo is slow as in the eusporangiates. Usually the epibasal half gives rise to the cotyledon, stem, and root while the hypobasal half forms the foot.
Apogamy and apospory are known in the Osmundales. The former takes place naturally and the latter from wounds. Aposporous gametophytes are diploid.
Thus the Osmundales, although resembling the leptosporangiates in some characters, resemble the eusporangiates in :
(1) the pseudoeusporangiate development of the sporangium which has a shield-shaped lateral annulus
(2) the stipular structure
(3) the morphology of the gametophyte
(4) the large number of sperms and their escape by breaking an opercular cell
(5) the relatively late differentiation of the embryo.