2 LICHENS OF THE STATE OF WASHINGTON 



fruticose. In crustose types the thallus grows flat upon the substratum, often 

 so closely attached that it is difficult or impossible to get it off without break- 

 ing the lichen. The thallus may be simply a tangled layer of interwoven 

 hyphae and algae cells with no differentiation into layers, or there may be 

 an upper pseudocortex with an algal layer below this and some indication 

 of a medullary layer below the algal layer. Hyphal rhizoids probably pene- 

 trate the substratum, although it is difficult to find these in a cross section 

 of the thallus. In some crustose species the thallus is entirely within the 

 substratum. In foliose Hchens the thallus is usually much larger, leaf-like as 

 the name suggests, not so closely attached to the substratum, and usually 

 divided into distinct layers. These layers are an upper dermis, an upper 

 cellular cortex, an algal layer, a medullary layer, and a lower cellular 

 cortex. However, some foliose lichens do not have all of these layers. In 

 fruticose types the thallus grows at right angles to the substratum. These 

 Hchens may be either pendant, hanging from the branches of trees, or grow- 

 ing upright from the substratum. They are often slender and long, some- 

 what hair-like, more or less cylindrical in form. The thallus has a pseudo- 

 cortex of interwoven hyphae, extending in the direction of the axis or at 

 right angles to it ; just inside is the algal layer and inside that the medullary 

 layer, which may be well developed, forming a solid center, poorly devel- 

 oped, or scarcely developed at all, leaving a hollow center. 



The fruit-bodies of lichens are either apothecia or perithecia. Apothecial- 

 bearing lichens are much more abundant than perithecial-bearing ones. 

 An apothecium is a cup-shaped or saucer-shaped body surrounded by either 

 a thalline or a proper margin (exciple) or by both. The epithecium, when 

 present, is the topmost layer of the disk, very thin and scarcely distin- 

 guishable from the hymenium, which lies beneath. The hymenium, or 

 thecium as it has been called by some students, consists of asci and para- 

 physes, and below it is the hypothecium. These margins (exciples) are 

 very important key characters for the separation of genera. The thalline 

 margin is continuous with the thallus, resembling it in structure, and always 

 contains some of the same algae that are found in the thallus. When present, 

 the proper margin lies just inside the thalline; it is an extension of the 

 hypothecium, and hence more truly a part of the apothecium than is the 

 thalline margin. A perithecium is an oval or spherical-shaped body, opening 

 only by a small pore at the top, which produces ascospores and has a struc- 

 ture very similar to an apothecium. 



As in free-living fungi, the ascospores produced in the fruit-bodies of 

 lichens must be the result of sexuality somewhere in the life cycle. More- 

 over, both sex organs have been seen in a few lichens, but the entire cycle 

 has not been satisfactorily worked out. No doubt the most common and 

 valuable method of reproduction is by means of soredia. A soredium is a 

 group of algal cells surrounded and held together by a mass of fungal 

 hyphae. Masses of soredia have the appearance of powder on the upper 

 surfaces of many lichens ; they are often produced in large quantities and 

 are disseminated by wind and rain. Often on the trunks of trees in the dense 

 forests west of the Cascades there can be seen thick layers of soredia on the 



