6 LICHEN FLORA OF THE UNITED STATES 



stipe, therefore, belongs to the sporogenous tissue, whereas the podetium has the 

 essential character of the thallus and more properly belongs to the vegetative tissue. 



Rhizoids and Cilia. The rhizoids are found on the ventral side of most 

 foliose thalli and serve as attaching organs. They appear to the eye as rootlike 

 bodies, varying in color from white to black. 



The cilia are like rhizoids in structure, but are found on the upper surface 

 of the thallus and along the margin. The hyphal rhizoids of the crustose lichens 

 are quite different morphologically. The functions of cilia are doubtless to retain 

 drops of water and gradually absorb them. Closely related to rhizoids is the 

 single attaching organ known as the umbilicus on the ventral sides of the thalli of 

 species of Gyrophora, Umbilicaria, and some species of Dermatocarpon. 



Other Structures. Spermagonia, soredia, cephalodia, and cyphellae are 

 structures which occur on or near the surface of thalli. The spermagonia are 

 supposedly the male reproductive organs, sometimes quite conspicuous as dark- 

 colored spots on the upper surface of the thallus, as in some species of Parmelia 

 and other large foliose lichens; but these structures are more often minute and of 

 the color of the thallus so that they appear only in sections. They were formerly 

 thought to be of considerable value in the determination of species. Their structure 

 and function will be further considered in the section on reproduction. The 

 soredia are small, powdery masses, usually whitish in color, and scattered over 

 the surface of the thallus as in Pyxine sorediata, and many other lichens. They 

 will be considered in succeeding sections. Cephalodia are wartlike bodies found 

 on the upper surface of the thallus as in Peltigera aphthosa, or within the thallus, 

 as in some other lichens. Cyphellae are small pits or depressions in the lower 

 surface of some foliose thalli as in some species of Sticta. Cephalodia and cyphellae 

 will be further considered under minute morphology, as their structure can be 

 made out only with the microscope. 



Finally, in some lichens the hypothallus is conspicuous to the unaided eye 

 or seen with the hand lens. This is true of some members of the family Panna- 

 riaceae. For instance, in Placynthium nigrum this structure appears as a bluish 

 black ring all around the thallus. Some of the older workers considered the rhizoids 

 a portion of the hypothallus. 



MINUTE MORPHOLOGY 



Foliose Thalli. In the foliose type of thallus there are the following layers : 

 an upper dermis, an upper plectenchymatous cortex, an algal layer, a medullary 

 layer, and a lower plectenchymatous cortex. The dermis, however, is scarcely 

 distinguishable in many foliose lichens, and there are several other variations from 

 the structure just outlined. For instance, in Collema there is no cortex, and the 

 algae are not arranged in a definite layer; in one genus of the Physciaceae the 

 plectenchymatous cortex is replaced by layers of densely interwoven hyphae; in 

 species of Peltigera the upper plectenchymatous cortex is present, but there is 

 none below; a few small foliose thalli, as in some species of Acaropsora, are 

 plectenchymatous throughout. Other instances of similar modifications in struc- 

 ture will be met in the descriptions of the genera. 



Crustose Thalli. In those crustose forms that are hypophloeodal or hypo- 

 lithic, there is simply a tangled layer of interwoven algal cells and fungal hyphae, 

 without any differentiation into layers. In the epiphloeodal and epilithic species 

 the structure may be quite as rudimentary, or there may be a more or less evident 

 upper pseudocortex of interwoven hyphae. In instances of the latter there is 



