Spore walls have been investigated in a number of pteridophyte species representing all of the major pteridophyte groups (reviewed in Lugardon 1990; Tryon and Lugardon 1991).
Spore wall development is well understood in the homosporous lycopsid Lycopodium clavatum (Uehara and Kurita 1991). Shortly after meiosis, the plasma membrane of the sporogenous cell folds into a pattern that later becomes the reticulate spore sculpture. Small WLCL form on the plasma membrane and accumulate in a centripetal fashion, forming the greater part of the exospore. After the main lamellate part of the exospores is formed, an inner granular layer, possibly derived from the spore cytoplasm, is deposited. In some Lycopodium there are no extra-exosporal layers (Uehara and Kurita 1991) whereas in others a thin extra-exosporal layer is deposited after the completion of the exospore (Tryon and Lugardon 1991).
Spore structure and development in heterosporous lycopsids differ between microspores and megaspores. In the clubmoss selaginella, microspores possess an exospore consisting of two layers (Fig. 3). The thin inner layer is the first to develop and comprises imbricate lamellae that are formed on WLCL in a centripetal direction (Tryon and Lugardon 1991). The outer layer starts to form only once the inner layer is complete. Some selaginella species may also develop a thin perispore or a paraexospore. In the microspores of the heterosporous lycopsid Isoetes japonica, a large gap is developed between the two exospore layers (Uehara et al. 1991). The outer exospore layer is regarded as a paraexospore as it begins to form before the inner exospore, consists of similar sporopollenin, and is completed at the same time as the inner exospore.Fig. 3
Proposed model of spore wall development in selaginella microspores. The thin inner exine layer forms first and comprises lamellae formed centripetally on WLCL. The outer exine starts to form once the inner layer is complete. Note the presence of callose at early developmental stages around the spore mother cell.
Selaginella megaspore walls contain two layers of similar thickness (Morbelli 1995). The inner and outer layers consist of lamellae and poorly segregated components, respectively. The inner layer does not thicken during exospore development and a dense basal layer is formed by the lamellae. In contrast, the outer layer increases significantly in thickness due to self-assembly (Hemsley et al. 1994, 2000; Gabarayeva 2000). During the final stages of sporogenesis, the endospore forms between the plasma membrane and the exospore. In Isoetes, the megaspore wall is similar to that of selaginella in terms of development and structure, consisting of two layers, with the formation of the outer layer commencing prior to that of the inner layer. Substantial quantities of silica are deposited within and on top of the outer layer before the exospore is completed. Finally, the endospore is laid down between the plasma membrane and the exospore.
The exospore in homosporous ferns develops centrifugally and is once again bilayered. The inner layer acts as a substructure and consists of varying numbers of fused sheets (extensive interconnected laminae) that form by sporopollenin accumulation on WLCL. The homogeneous outer layer is considerably thicker and contains thin radial fissures and small cavities. An extra-exosporal layer (perispore) forms once the exospore is complete and is deposited from the decaying tapetum. Spore wall development in heterosporous ferns is similar to that observed in homosporous ferns, and is also similar in both microspores and megaspores.
In sphenopsids the spore walls appear to be highly derived (Lugardon 1990), and observations of Equisetum arvense have shown that four layers are present in the form of an exospore, an endospore, a middle layer and pseudoelators (Uehara and Kurita 1989). The exospores comprise inner and outer exospores. The broad and homogeneous inner exospore forms first by way of plate-like structures accumulating on the plasma membrane. The outer exospore is then formed by the deposition of granular material on the inner exospore and is similarly wide and homogeneous. Once exospore formation is complete, the middle layer forms in the gap between the exospore and the plasma membrane. The pseudoelators are the next structure to form and consist of two layers. The inner layer comprises longitudinal microfibrils during the early stages of development but eventually becomes homogeneous. The outer layer is also homogeneous and is formed by granules that are released from vesicles in the plasmodial cytoplasm. The pseudoelators are connected to the spore, by way of the middle layer, at the aperture. The endospore is the final component of the wall to form on the inside of the exospores (Taylor 1986; Uehara and Kurita 1989).