THE LICHEN-FLORA OF CHICAGO AND VICINITY. 11 



THE THALLUS AND APOTHECIUM SOME OF THEIR ORGANS. 



The thallus, when present, and the apothecium, are to be con- 

 sidered and studied externally, but the organs of the plant must 

 be examined with the microscope. Some of these have been 

 mentioned, and I can only refer briefly to others, doing so in order 

 to guide the first steps of the explorer, who, at every advance, will 

 meet with new problems and find all aids necessary. In exami- 

 nation, the membraneous thallus, cut in a thin section, will show 

 above what is known as the cortical layer of minute aggregated 

 cellules; next, the gonidial layer of green cells; then will follow 

 compacted filaments (hyphse), and in this formation rest the goni- 

 dia. Below is the inferior cortical layer, from which, in many species, 

 proceed fibrils (hypothallus), attaching the plant to its substrace. 

 Some lichens, such as the parasitic, and some others, Peltigera and 

 CoUema, have no thallus; either the upper or lower cortical layer 

 may be absent, or but one scantily represented. The gonidial 

 and other wart-like eruptions sometimes appearing on the crust 

 (soredia), constituted the basis of the old pseudo-genera. The 

 color of the thallus above and below may differ, as in Sticta. The 

 gonidia of divers forms and colors are variously placed, singly or 

 grouped, or of confervae-like character.* 



The apothecium, which has been slightly noticed, whatever its 

 shape, has a uniformity of internal structure. The disk-epithecium, 

 (exciple), contains the other organs, and is enclosed by a margin, 

 which may be formed from the thallus (thalline margin). It is 

 then known as lecanorine. When the margin or border is not 

 thalline the apothecium is lecideine. Then there is the nuc- 

 eliform apothecium of angiocarpous lichens. The exciple, when 

 colored or blackened, is biatorine; when composite, zeorine. 

 Grouped apothecia form a stroma. The perithecium and 

 amphithecium of the angiocarpous lichens are the outer and 

 inner coverings; within is the nucleus. A section of the apothecium 

 viewed under the lens, shows the epithecium to be of colored, 

 granular matter and borne on the apices of what are termed the 

 paraphyses — slender filaments growing from the medullary layer. 

 The apices are globular or clavate. The thalline exciple contain- 

 ing gonidia, and the hymenium, bearing the thekes, within which 

 are the spores, will be seen. The hymenium is either colored or 



*Th. M. Fries, Dr. Minks and others as to their systematic value. 



