Development Of Spore Wall In Main Spore Types Of Polypodiales From China

Polypodiales (Filicales) is the largest group in pteridophytes, and it is a group, which is the most difficult to classify. The morphology of the fern spores is stable and their ornamentations have important values in the systematic evolution and taxonomy of pteridophytes. There are many reports about the relationship between the surface ornamentation types of pteridophytes and the taxonomy. However, there are few reports about the development of spore wall and the formation of the spores of pteridophytes. The formation and development of fern spores are an important phase in the life cycle of fern; it has important value in systematics of pteridophytes to study the origin and formation of ornamentation of fern spores, and to study the developmental mechanism of the spore wall.In the present paper, the spore development, formation process of spore wall and its ormamentations of seven species belonging to Polypodiales and distributed in China, i.e., Ceratopteris thalictroides (L.) Brongn., Pteris vittata L., Coniogramme taibaishanensis Ching et Hsieh, Lygodium japonicum (Thunb.) Sw., Stenoloma chusamum Ching, Lepisorus thunbergianus (Kaulf.) Ching, and Cyrtomium fortunei J. Sm., which represent the Ceratopteroid type, Pteroid type, Adiantoid type, Lygodoid type, Vittaroid type, Polypodoid type and Dryopteroid type, respectively, were studied mainly by using transmission electron microscope (TEM) and combining with the observation of scanning electron microscope (SEM) and light microscope (LM). The present work offers the basic information for the developmental biology and palynology of pteridophytes.The spore wall development of seven species was investigated. The results are as follows:[1] The sporangium is initiated from a single surface cell which developed to form a sporangium by mitosis. The young sporangium consists of a layer of sporangial wall cells, two layers of periplasmism tapetum cells and the inside sporocytes. When the coat membrane around the sporocytes split into two, the rectangle sporocytes separate from each other and become rounded gradually. At the same time the cell wall of the inner layer of tapetum begin to break down. And the protoplasts fused and intrude among the sporocytes.[2] The sporocytes form tetraspores by meiosis and enter the tetrad phase. The radially symmetrical trilete spores result from the simultaneous tetrahedral divisions and the bilateral monolete spores result from successive non-tetrahedral divisions. In the process of the monolete spore formation, the arrangement of the tetraspores has two fashions. One is that the two daughter spores of the tetraspore are perpendicular to the other two, the other is that the two daughter spores are parallel to the others. After cytokinesis, each of the tetraspores was enveloped with a transparent plasmolemma and covered by coat membranes which separate the tetraspore from each other.[3] The exospore can be generally distinguished into two layers, i.e., the inner layer of exospore and outer layer of exospore. The inner layer is derived from the protoplasm of the spore. It is a thin structure which was deposited rapidly on the surface of the plasmalemma. The outer layer is derived from the inner layer of tapetal cells. It is a relatively thick structure and formed bythe small flakelet and granules which deposite in centrifugal direction.[4] The laesura (i.e., aperture of spore) is a fold which is formed by the contracting of the protoplast in the site of the joint of tetraspores as the exospore forms. Thus it is also called tetrad traces. In the process of the exospore formation, the sporopollenin is deposited relatively less and late on the apex of the laesura. Due to the difference of the meiosis, the laesura can be divided into two types, i.e., the monolete and the trilete.[5] The perispore exists in all species of Polypodiales. It is formed on the surface of exospore in centrifugal direction after the separation of tetraspores. It is derived from the tapetal cells and sporangial wall cells. In Polypodiales, the perispore is the most co...