INTRODUCTION 11 



tibly gelatinized. Aside from the functions already named, the hyphae of the 

 medulla must carry up to the algae any materials taken in a crude state from the 

 substratum by the attaching organs. Also the foods assimilated by the algae must 

 in part pass downward or inward by means of these hyphae for the nourishment 

 of certain portions of the thalli. It is also supposed that fats and other food 

 materials are often stored in the hyphae of the medulla. 



The Lower Cortex. In most respects the lower cortex is very similar to 

 the upper, but, as already stated, it is neither so constantly present nor is it as a 

 rule so thick. When the lower cortex is present the rhizoids extend from it into 

 the substratum, but when it is absent rhizoids are usually present as continuations 

 of certain hyphae of the medullary layer. Plainly, such a plectenchymatous tissue 

 on the lower side of the thallus must serve for support, and it becomes very thick 

 in those large foliose thalli which are attached by an umbilicus, such as species 

 of Gyrophora, Umbilicaria, and Dermatocarpon. But, since it lies between the 

 rhizoids and the tissue above, this cortical tissue must permit the passage of any 

 crude or elaborated materials taken from the substratum by the rhizoids, for which 

 reason it remains thinner than might otherwise be expected. Also, the lower cortex 

 is usually a looser tissue than the upper cortex and is usually darker colored. 

 Peltigera is a genus of large foliose lichens in which there is a well-developed upper 

 cortex and no lower cortex. 



Soredia and Spermagonia. A soredium consists of a tangled mass of algal 

 cells and hyphae which has pushed upward from the algal layer and appears at the 

 surface in a small spot where the upper cortex is broken. The soredia function for 

 reproduction. The spermagonia appear as little spherical or cup-shaped bodies, 

 sunken in the surface of the thallus. Within each spermagonium are borne the 

 spermatia on stalks called sterigmata. The spermatium is a small, slender body, 

 commonly colorless, and straight or slightly curved. The spermatia are usually re- 

 garded as male reproductive bodies. 



Cephalodia. These bodies (Fig. 3) sometimes appear upon the surface of 

 the thallus and in other instances within it. Those situated on the upper or lower 

 surface are called ectotrophic, and those that are found within the thallus endo- 

 trophic cephalodia. There is much difference of opinion as to the frequency of the 

 appearance of cephalodia, due partly to lack of careful observation and partly also 

 to difference of view as to their nature. A cephalodium is an abnormal develop- 

 ment upon or within the lichen thallus, containing some other alga than the one 

 found in other portions of the thallus. The cephalodia of Peltigera aphthosa are 

 ectotrophic and plainly visible. The foreign algae found in the cephalodia of this 

 lichen are of the genus Rivularia, and they are supposed to be brought to the 

 thallus in some way, and, being held by the trichomatic hyphae, to multiply and 

 become surrounded by a true cortex. Ectotrophic cephalodia also occur in species 

 of Stereocaulon, and the algae contained are not always the same. But these are 

 minute structures and seldom noticed. Endotrophic cephalodia are said to occur 

 sometimes in Sticta amplissima. 



Cyphellae. These structures (Fig. 4) are small pits found on the lower side 

 of certain thalli. They are common in the majority of species of Sticta, and 

 appear at points on the lower surface of the lichen thallus corresponding to the 

 position of breathing pores. In their development a circular break first appears 

 in the lower cortex. As the opening in the cortex increases in size, the adjacent 

 hyphae of the medullary layer give off numerous short branches, which fill the 

 bottom of the opening so that the cyphella appears as a minute cup-shaped or 



