INTRODUCTION 7 



frequently also a more or less distinct algal layer below the pseudocortex and 

 some representation of a medulla below the algal layer. Whether these superficial 

 forms are thus differentiated or not, the modified hyphae known as hyphal rhizoids 

 may always be looked for, penetrating the substratum. It is, however, very diffi- 

 cult to observe them in sections of these thalli. With each description of the 

 crustose genera will be found a statement regarding the amount of differentiation. 



Fruttcose Thalli. The fruticose type of lichen thallus is peculiar, and is 

 for this reason treated after the crustose type. Fruticose thalli are usually, though 

 as noted below not always, more or less cylindrical in form. The outer layer 

 of the cylinder or other form is a pseudocortex of densely interwoven hyphae, 

 extending either in the direction of the axis or at right angles to it. Within this 

 outer layer is the algal layer, which, like the outer layer of hyphae, is commonly 

 more or less nearly circular in transverse section. Sometimes, as in species of 

 Alectoria, there is within the algal layer a well-developed medullary layer filling 

 the remainder of the space and making the cylinder a solid one. In other instances 

 the medulla is poorly or scarcely at all developed. Within it, when only partially 

 developed or within the algal layer when the medulla is wanting, is frequently 

 found a solid or a hollow cylinder of hyphae extending in a longitudinal direction. 

 If this cylinder of hyphae is hollow there are usually strengthening bundles of 

 hyphae to be found traversing the hollow central portion of the thallus in trans- 

 verse direction. 



The outer pseudocortex is usually composed of hyphae that are more or less 

 gelatinized, so that the structure is very difficult to make out. This gelatinization 

 no doubt increases the protective function of the pseudocortex. The inner cylinder 

 of hyphae functions principally for conduction like a stele, and the walls of the 

 hyphae show much less gelatinization. The algal layer is seldom a complete 

 cylinder, the algae more often appearing in clusters incompletely filling the space. 

 The pseudocortex is seldom even approximately of the same thickness throughout 

 and is sometimes scarcely at all developed except over the algal clusters. This 

 arrangement brings the algal layer into closer contact with the atmosphere without 

 completely exposing the algae. The hyphae are much less frequently branched in 

 fruticose thalli than in foliose and crustose forms. In connection with this fact 

 it is to be noted that fruticose thalli are not always even approximately cylindrical 

 in form. Ramalina calicaris fraxinea departs most widely from the cylindrical 

 form and appears much more like a foliose thallus growing away from the sub- 

 stratum, to which it is attached at a single point. However, when this thallus is 

 sectioned, the structure is seen to be essentially that of the fruticose type. Indeed, 

 in outward form, there is every gradation between the fruticose and the foliose 

 thallus, and in a few so-called fruticose thalli there is found plectenchymatous 

 cortex characteristic of a foliose thallus. 



The Dermis. This structure consists of a few layers of flattened plectenchy- 

 ma, lying upon the cortex and derived from it. The dermis aids in protection 

 against excessive evaporation of moisture, and from its surface there sometimes 

 arises a dense covering of short hyphae which also function in the same way. 

 These are the trichomatic hyphae of certain species of Peltigera, and they serve 

 in Peltigera aphthosa to help in retaining the soredia which develop into cepha- 

 lodia. The dermis is rudimentary or wanting in most lichens except species of 

 Peltigera and Sticta. The inner layers of plectenchyma are less flattened and 

 gradually pass into those of the cortex from which they are developed. 



The Upper Cortex. This structure is plectenchymatous, consisting usually 

 of several layers. The cells of these layers may be as distinct as those of any 



