LICHENS, li-kenz (Lat. lichen, lichen, Gr. Reix0), a large but artificial group of the higher fungi (Carpophyta), characterized by parasitic growth upon the lower blue-green and yellow-green algae (Protophyta, Chlorophy cede). Lichens are of the wildest occurrence in nature, appearing as gray, yellow and brown crusts or masses almost everywhere upon trees, rocks and soil. The number of genera and species differs more or less with the authority cited: the valid genera number not far from 250, while the species are in the neighborhood of 4,000.
The vegetative body or thallus varies from a fraction of a millimeter to several decimeters in size, though it shows relatively little varia tion in thickness. In texture it is powdery, leathery, paper-like, or, in the case of many forms with blue-green algae, gelatinous; the prevailing colors are gray, brown and yellow, while green and black sometimes occur. The shape of the thallus is typically orbicular or stellate; it is often irregular, especially in branched forms. In general appearance the thallus varies within wide limits; as a rule, however, three types, crustose, foliose and fruticose, may be clearly divinguished. The crustose type is the primitive one, showing in its granular, warty and areolate forms the various stages through which the thallus has passed in its development from the original mycelium. The crustose thallus is so closely in contact with its substratum that it cannot be separated from it without tearing. The foliose type is a higher development of the crustose. It is usually a definite, leaf-like structure, more or less lobed at the margin and attached to the stratum somewhat loosely or at but a single point. The fruticose type is a special modification of the foliose, in which the latter is more or less flattened or cylindrical and erect or pendulous. This form is prob ably an adjustment to conditions of diffuse light. It is especially characteristic of tree lichens and of certain ground forms, such as Cladonia, where it is termed the podetium. In the latter there is also developed an accessory or secondary thallus, consisting of minute, leaf like scales.
The simplest thallus consists merely of a few fungus threads enclosing the irregularly dis posed cells of the host or alga. Ordinarily, however, the algal cells not only have a definite position, but the fungal portion of the thallus is likewise highly specialized. Naturally, this dif
ferentiation is least in the crustose forms and greatest in the fruticose ones. The structure of the foliose type may be taken as fairly rep resentative, except of the gelatinous lichens, in which the alg are scattered throughout the thallus. A definite epidermal layer is wanting except in a few of the higher lichens, where the outer filaments have been gelatinized, resulting in the formation of a structure closely resem bling a cuticle. As a rule, however, the upper most part of the thallus is the cortical layer. This consists of hyphae (filaments) compacted in such a way as to produce a tissue which looks much like parenchyma and is called in conse quence pseudoparenchyma. The function of the cortical layer is in part mechanical or sup portive and in part protective. Its structure seems to depend primarily upon the latter func tion: it is least in those forms growing in for ests and greatest in those found in the open. Below the cortical layer and continuous with it is found the host or algal layer, consisting of filaments more or less loosely intertwined with the algae. This is the nutritive layer, in which the fungal .hyphae draw their nourishment from the host-cells. The connection between the two may be merely by contact or by penetration. In the latter case the fungal hyphae eitherpene trate the protoplasm of the host and finally destroy it, or merely pierce the cell-membrane and lie in contact with the protoplasm. In either event, the hyphae develop special branches for contact or penetration, which are called haustoria. The algal layer is a specialized por tion of the medulla which lies just below it. The hyphie of the two layers are continuous, but they do not develop haustoria in the me dulla, where they tend also to run more or less parallel with the direction of growth. The med ullary layer primarily serves the function of transport; it is likewise used for the storage of lichenin (lichen-starch) and fats. The lower surface of the thallus is covered with a cortical layer similar in structure to that of the upper surface. Generally, however, it is somewhat thinner and is designed rather for absorption than for protection. It is frequently produced into facicles of hyphae termed rhizoids and cilia.