The Biological Significance of Morphological Characters in Bryophytes: The Spore

Aspects of standard methods for obtaining information about spore size are exemplified by measurements in Ceratodon purpureus. Principles for com- paring spore sizes of species are discussed, and emphasis is given to the criteria for selection of material. The significance of spore surface sculpturing is unknown; one function may be to prevent, by interaction with the surface tension of water, small quantities of water from activating spore germination. Fully developed chloroplasts are usually present in bryophyte spores, however reduction in chloroplast numbers and struc- tures may occur towards spore maturity. Unicellular spores are presumably de- rived from multicellular spores by blocking of the nucleus in the G,-phase during sporogenesis. The sporadic occurrence of multicellular spores within certain groups indicates that unblocking may also occur. Isospory, heterospory, anisospory and pseudoanisospory (false anisospory) are distinguished from plurispory, amphispory, and combispory, three new terms pro- posed in emendation of names for the seven phenomena. Anisospory causes differential dispersal potentials between female and male spores, and it serves to maintain variability of a taxon by back-crossing to the female genotypes; this gives emphasis to the importance of recessive alleles in phenotypic variation. Pseudoanisospory is different from anisospory and carries much resemblance to genetic systems in which homozygotes are inferior to het- erozygotes; the main biological effect of this phenomenon is to maintain variability by balanced polymorphism. The minuteness of spores and the large quantities in which they may be produced holds a fascination of its own, and only through concerted efforts in a multitude of research disciplines can our understanding be expanded. A great many data have accumulated in the past, but new methods or changes of old methods have thrown some doubt on their applicability. As some data result from field observations and others from experiments in the laboratory, the concerted efforts, as metaphorically compared with an orchestra, pres- ent a sound much too defective. I have applied simplifications in some cases to show principles, and therefore a warning against too rigorous application of some conclusions must be given. The relationships between field work, laboratory experiments and creation of new theories should be kept in mind when reading this paper. The theoretician deals with all conceivable worlds, the laboratory workers deals with all possible worlds, and the field worker deals with the real world. The laboratory worker must ask the theoretician if his world is an interesting one, but he must also ask the field worker if it is related at all to the real world. The study of bryophyte spores is still in its infancy, and very few scientists-if any-are able to claim a broad expertise in this real world.