Dimorphism

Dimorphism. 1. Certain minerals and chemical substances possess the property of crystallising in two or more distinct forms. When two forms occur the substance is said to be dimorphous, and the phenomenon is known as dinwrphism; in the case of three forms, it is trimorphous, or the general term, pleomorphous, may be applied. Thus sulphur crystallises in either rhombic crystals or monoclinic (crystallography); it is hence dimorphous. Calcium carbonate is also dimorphous, existing both as calcite and aragonite. The oxide of titanium is an example of a trimorphous body, occurring in the three forms known as rutile, anatase, and brookite.

2. Different individuals of the same species often present very dissimilar form and structure. This is known as dimorphism, a phenomenon of which there are three main varieties, viz. seasonal, sexual, and functional. Seasonal dimorphism is best shown in regions which have a much greater difference between the summer and winter climates than in our temperate country: good illustrations are afforded by the annual change in colour of many northern animals, such as the Arctic fox, the Scandinavian grouse, etc., which are brown in summer and white in winter. Sexual dimorphism is more frequent; the differences between the peacock and the peahen, the lion and the lioness, the horned ram and hornless ewe, the occurrence of antlers among deer and reindeer only in the male, etc., are all cases of this variety. The differences between the two dimorphic forms in these cases are, however, insignificant when compared with those which occur among invertebrates; perhaps the most striking of these are met with among the Cirripedia (q.v.), where tbfe male is a dwarfed form, very imperfectly developed, and which lives only to fertilise the female. A very familiar case among Invertebrates is that of the Bees (q.v.), where there are two kinds of females - the barren workers and the fertile queen bees. Functional metamorphism is probably the most important; it occurs mainly among colonial animals. The Bryozoa afford good illustrations both of colonial and individual dimorphism; in the former it may be due directly to the action of the environment on the growing colony. Thus one species of Bryozoa may occur in the "Lepralian" form, as a mere crust over shells or stems; or it may rise into foliaceous frond-like expansions in the "Escharine" foim, or it may form cylindrical branching shoots in the "Vincularian" form. In addition to this the individual zooids of the colony may exhibit functional dimorphism. Thus some zooids may be modified into the prehensile organs known as "avicularia," others into locomotor organs as vibracula, others into ovicells (marsupial pouches), and others into stems or root fibres. In cases where there are so rnany modifications, the phenomenon is generally referred to as "polymorphism." Another good illustration of this is afforded by the Siphonophora (q.v.) where the float or pneumatophore, the protective bracts or hydrophyllia, the rectocalyces, etc., are all dimorphic, or rather polymorphic, forms of one type of polypite. [Heterogony.]