4 LICHEN FLORA OF THE UNITED STATES 



are in part or wholly epiphloeodal or epilithic. Others are scurfy or granular, and 

 these are usually rather poorly developed and thin. In thicker forms is found 

 the warty or verrucose condition; sometimes there occur here and there minute 

 chinks, so that the thallus is said to be rimose or chinky; or finally the chinks 

 may become numerous and divide the thallus into minute or small, several-sided 

 areas, known as areoles, in which case the thallus is said to be areolate. 



Colors of Thalli. As compared with size and form, color is usually re- 

 garded as a rather more variable and therefore less reliable taxonomic character. 

 Yet the colors of thalli play an important part in identifying lichens and, though 

 often quite variable, they must be carefully noted. Colors in lichen thalli vary 

 all the way from a white to a black, but the most common is a greenish gray. 

 Some other colors which occur are ashy, olivaceous, brown, and straw-color, 

 as well as yellow and various intermediate shades, such as brownish black, 

 olive-brown, etc. The thallus, furthermore, is often variegated and the lower 

 surface is frequently of a different color from the upper. Also, in the fruticose 

 forms the basal portion is frequently of a different color from the distal portions, 

 usually darker. The tendency of thalli, as of other lichen structures, is to darken 

 with age, and the variations of color in a species may usually be traced to peculiar 

 conditions of growth. 



The Ascocarp. In the fruit, or ascocarp, the main features of gross mor- 

 phology are size, form, and color. The ascocarps are usually superficial and large 

 enough to be seen easily with the unaided eye. In some lichens, however, they 

 are so small that they can be made out only with difficulty with the hand lens; or 

 they may be immersed in the thallus and indicated at the surface by a slight 

 elevation or depression as a disk or an ostiole; or, when immersed, they may be 

 scarcely discernible in any way except in sections of the thallus. From 0.1 to 

 5 mm. is well within the usual range for diameters of the ascocarps, though larger 

 ones will be found. 



The ascocarp may be an open or disk-shaped body called an apothecium. 

 The apothecia are most commonly saucer-shaped, or some slight modification of 

 this form, as when the disk is flat or somewhat convex instead of concave. In 

 some instances the disk becomes very concave, the apothecium at last becoming 

 cup-shaped, or it may be strongly convex, sometimes becoming at maturity 

 spheroidal. In all of these forms the outline of a transverse section of the 

 apothecium when young would usually be very nearly a perfect circle; but the 

 form may become irregular as growth proceeds, so that at maturity this outline 

 is quite irregular. In other lichens the apothecia are non-circular from the be- 

 ginning. Thus, there are the elongated and often-branched forms, such as are 

 found in the Graphidaceae, and the difform or variously irregular forms, as in 

 the Arthoniaceae. 



The ascocarp may be closed, with only a small opening at the top for the 

 escape of spores. It is then called a perithecium and usually approximates a spher- 

 ical form, inclosed by a more or less well-developed perithecial wall. 



The Disk. In those lichens in which the exciple is not produced into a 

 perithecial wall the upper surface of the ascocarp is naked, except for a very thin 

 film of thallus which may persist as an epithecium, a structure not mentioned in 

 the descriptions of species. This upper and essentially naked surface, whether 

 flat or more or less strongly concave or convex, forms the disk. The outline of 

 the disk, then, may be circular or variously elongated or irregular, varying in this 

 respect with the form of the ascocarp as a whole. In color the disk varies con- 



