Pleuridium

Spore ornamentation in the genus Pleuridium View in CoL

In order to compare a new species with other similar Pleuridium species from Eurasia we undertook SEM study of spore ornamentation ( Figs. 6–8). Surprizingly, bacula in the premature, the fully and optimally developed, and the fully but suboptimally developed spores appeared to be sometimes different.

Pleuridium subulatum spores were especially polymorphous in different populations ( Fig. 6), although the spores from any single well-developed capsule were rather homogeneous in their ornamentation. The shape of bacula are somewhat dependent on the gold coating mode. Not coated spores have bacula of the equal width to sligtly broadened above and thus somewhat clavate ( Fig. 6C, D). However, after 5 nm coating with gold ( Fig. 6E), their caputs appeared to be more abruptly delimited. A similarly ornamented spores from the nearby locality ( Fig. 6A–B), coated with ca. 30 nm of gold were found to be still narrower. The bacula are anastomosing at bases by low ridges ( Fig. 6A, E), and small granules are scattered among these ridges. Bacula bases are sometimes split and some bacula look raised on short roots ( Fig. 6A). Bacula tips are either solid and smooth ( Fig. 6C–E) or forked and covered by sparse granules ( Fig. 6A–B). Having a relatively limited observation, we refrain from more detailed explanation of effects of a sample preparation. However, drastic variation between various collections shown in Fig. 6 requires at least a preliminary hypothesis if the observed difference discloses a genetic variation or not. SEM and light microscopy studies may be interpreted that the main reason for this variation is a moisture condition during the sporophyte development. Plants collected in hemiboreal forest zone, where sporophytes of P. subulatum are developing in cold and wet conditions, have spores with high and thin bacula or clavae ( Fig. 6A–E). Some bacula from these areas form arches (arrowed in Figs. 6B, E), which are not seen in other samples. In more southern population, in oak forests vegetational zone, where plants experience periodocal drought during spore maturation, the bacula are shorter and thicker ( Fig. 6F–H), and ultimately, in steppe environments, the spore ornamentation is compised of scattered verrucae ( Fig. 6I–J). Similarly to the latter, spores taken from Polish plants of P. subulatum , illustrated by Deguchi et al. (1994) with SEM images, also have scattered granulae or verrucae. Small round granulae in between bacula or verrucae occur more or less regularly on the surface of spores from all studied collections. Currently we are inclined to explain the difference in spore ornamentation by the developmental conditions, but this might have different causes, and this intriguing question certainly requires further study.

Such wide variation in ornamentation of spore s of P. subulatum might look unique, however Carrión et al. (1990) described another example of quite strong variation, also with TEM study, observed in Tortula acaulon Hedw. (as Phascum cuspidatum ), the species that often grows with Pleuridium subulatum in a similar habitats and also occurs in various climatic regions.

Spores of P. aciminatum ( Fig. 7A–D) are covered with more dense and stout bacula, raised on much higher roots, which are more strongly anastomosed as it is seen in European specimen ( Fig. 7A–B), while obscured in the North American one because of dense bacula arrangement. Small granulae on the spore surface are seen between anastomoses, in a way similar to P. subulatum . Granulae on bacula tips are larger and more numerous, often forming conspicuous clusters, which are principally similar in two studied samples, from Europe ( Fig. 7A– B) and North America ( Fig. 7C–D).

Spores of Pleuridium japonicum were studied from two capsules of one specimen, but they are very different from other spores ( Fig. 7E–I). Spores from one capsule are clearly heteropolar ( Fig. 7E–G), possessing collarlike bacula arrangement at the proximal pole, and also the branches of their high bacula are fused by two or four, thus looking as a columns slightly cut at their ends. These spores were taken from a premature capsule, still greenish and somewhat pellucide. Another spore from the same collection has ornamentation much more similar to other species of Pleuridium , mainly because of small globules covering distal parts of bacula ( Fig. 7I). The mentioned aspect of premature spores, where bacula look slightly cut at their ends is not well seen in mature spores, probably because of bacula branching in the course of maturation.

Baculi on spore surfaces of P. kamchaticum ( Fig. 8A– D) are massive, regular, but with slight constrictions along their length, and terminate with a cluster of granulae, somewhat similar to those in P. acuminatum and P. subulatum . However, the bacula of P. kamchaticum are nev- er unbracheded. Granulae on the spore surface are seen between bacula.

Most similar to P. kamchaticum pattern of spore ornamentation was observed by us in P. palustre ( Fig. 8E– H); two different samples from Europe were used for that. Their baculi lack constrictions, observed in P. kamchaticum , but otherwise are very similar. In addition to similarity in spore ornamentation, P. kamchaticum is similar to P. palustre in capsule shape, which is broadest markedly below the middle (cf. Fig. 1A), although unlike the latter, its calyptra is cucullate and stem possesses a central strand.