The thallus of many lichens exhibits several peculiar structures, which are the direct result of the symbiosis of fungus and alga. The most frequent and most important of these is the soredium. This is a minute irregular mass of fungal hyphen and algal cells, readily carried by wind or water, and able to grow directly into a lichen thallus under the proper conditions of moisture and warmth. Soredia occur upon the upper face of the thallus of many lichens as elevated powdery masses or tubercles. They arise in the algal layer of the thallus by the re peated branching of a fungal filament in such fashion as to completely enclose one or more cells of the alga, which also increase in number. The hyphae become more or less gelatinized and compacted into a surface very resistant to desic cation. The soredia are pushed upward through the thallus by the growth of the filaments below and are finally extruded through a rift in the cortical layer, constituting a sorus. Normally, the soredia are carried away from the sorns and develop independently, but in some cases they grow while still in contact with ' the mother thallus, producing minute, leaf-like scalesup9n i the latter. These are the so-called isidioid growths or phylloclades, found in Usnea and related genera. Soralia are structures which arise from the medulla or even from the lower most layer by the upgrowth of a mass of par allel filaments which penetrate the algal layer and there develop into normal soredia. Cepha lodia bear the general appearance of soredia, but in origin and function they are quite differ ent. They are distinguished as external and internal. The cause of their development is unknown: they are said to arise from the soredia of other lichens, which have lodged upon the thallus.. They have never been produced ex perimentally, however, and it seems much more probable that they are modifications of the thallus due to a change in the life form of the algal element. The cyphellm are flat or concave gaps in the lower cortical layer of the thallus of Sticta and Stictina. They are filled with variously branched hyphz of the medullary layer and probably function as organs of ab sorption and respiration. The spermagonia are minute black dots occurring on the upper face of many lichens, especially near the margin of the thallus. Structurally they are identical with those propagative organs of black fungi that are termed pycnidia. They are spherical bodies with a membranous or carbonaceous envelope, containing a layer of rod-like filaments which bear at their tips tiny spore-like bodies called spermatia. As the names indicate, the sper magonia were supposed to be male reproductive organs and the spermatia the fertilizing cells. There now seems to be little doubt, however, that they are propagative organs or pycnidia inherited from fungus ancestors. In a few cases they may be pycnidial parasites, 'such as Phyllosticta.
The fruit (sporocarp) of those lichens which bear spore-sacs (ascolichens) is called a peri thecium when the fungus is one of the black fungi (Pyrenomycetates), a hysterothecium, when it belongs to the cleft fungi (Hysteriales), and an apothecium when the lichen is a cup fungus (Discomycetales). All these agree es sentially in structure, though they differ in form: the perithecium is globoid, opening by a minute pore at the top, the hysterothecium, linear or irregular, opening by a cleft, and the apothecium usually open and disc-shaped. The essential parts in each are the same, namely, spore-sacs (asci), the spores, and the sterile threads (paraphyses). The apothecium is the highest type of spore-fruit and the most widely distributed. It consists usually of the following
parts: the thecium, the central portion consist ing of the asci, sometimes called thekes, and the paraphyses; the epithecium, which lies above the asci; and the hypothecium which is found be low them. The latter often extends around the sides of the thecium also, and is there termed the exciple (proper exciple, parathecium). All of these consist of densely compacted elongate hyphm (prosenchyma) which are without algae and are often more or less dark colored. In most of the higher lichens the exciple is sur rounded by the tissue of the algal layer, .pro ducing a thalline margin (thalline exciple) about the apothecium. The apothecium usually sits directly upon the thallus; occasionally it is stalked, and less frequently it is immersed or innate. Lichens show the same differences with respect to paraphyses, asci and spores that are to be found among the other cup-fungi and black fungi. The paraphyses are simple or re peatedly branched, continuous or septate, gelat inized or non-gelatinized, persistent or evanes cent. They are often compacted and dark colored at the tip, appearing to be continuous with the epithecium. The spore-sacs of lichens are cylindrical or calvate in form, more rarely obovoid. Their walls are thin, though some times gelatinized; they do not react to iodine as a rule, except when very The asci usually open by a terminal slit ; in a few cases the entire wall breaks away. The number of spores in an ascus varies from one (Pertusdria) i to many (Acarospora); the normal number is eight, six or four. The arrangement of the spores is usually irregular, though they are sometimes in one row (monostichous), or in two rows (distichous). Lichen spores are or dinarily colorless and simple, less frequently dark brown and many-celled. They may be two-celled (bilocular), several-celled (pluriloc ular), or muriform, when the partitions run in both directions. The wall of the spore is smooth and without appendages; the contents may be hyaline, granular or guttulate.
Physiology and The func tions of lichens are essentially those involved in the relation between par a-)ite and host, modified to an important degree in those forms with well-developed thallus. Absorption of water takes place readily upon both surfaces of the thallus, but especially on the under side, where hyphm, cilia and rhizoids all act more or less efficiently as absorptive agents. According to Zukal, the hyphal hairs are capable also of absorbing moisture directly from the air, in Physcia, Peltigera, Sticta, etc. Water-storage takes place in the algal and medullary layers. It is effected primarily by the algal membranes, especially of the blue-green slimes, and to a degree also by the lichemn of the hyphm. Zukal has suggested that the cephalodia, be cause of their blue-green algae, are probably to be regarded as structures for the storage of water. The ability of the lichen thallus to re tain water arises from its complexity, and from the of the gelatinized cortex. Lichens exhibit a number of somewhat primitive devices for the exchange of gases. These are often mere rifts in the thallus, or degenerate pycnidia; sometimes definite openings are present, or absorption takes place through loose protuber ances. A specialized organ. for this purpose is found in the cyphellw of Sticta and Stictina, which are primitive breathing pores, making direct connection between the air and the medullary layer. The latter serves as a path way for the transport of water and gases to the various parts of the thallus.