8 LICHEN FLORA OF THE UNITED STATES 



ordinary parenchyma or the walls may be gelatinized and swollen to such an 

 extent that the cellular nature is not made out. This layer may appear quite 

 hyaline in section or it may contain some coloring matter. In the majority of the 

 foliose lichens, such a structure is developed both above and below, and the lower 

 cortex is more frequently of a dark color. However, as already stated, there are 

 a number of foliose genera in which part or all of the species lack such a cortex 

 on one or both sides. And it may be added here that in species of Leptogium the 

 cortex usually consists of a single layer of plectenchyma. 



The cortex is constantly being built up from the hyphae of the algal layer 

 below and is gradually transformed above into a dermis. By this gradual upward 

 passage of tissues dead entangled, algal cells are at length carried off by the 

 abrasion of the upper surface. The lower cortex is usually thinner and is 

 more often absent than the upper; but in thalli in which this lower cortex is 

 especially needed for mechanical support it is often better developed and thicker 

 than the upper cortex. 



The Algal Layer. In some lichens, as in species of Collema, the fungal 

 hyphae are simply in contact with the algae and in others they are attached 

 to them by haustoria. In some cases the haustoria penetrate the walls of the algal 

 cells and are then said to be intracellular. These naturally secure the closest 

 union. Every gradation between intracellular and extracellular haustoria may be 

 looked for in certain species, but it is rarely that the haustorium attains a full 

 development, i.e., is divided into a network of hyphae, within the algal cells. 

 Indeed, intracellular haustoria are either very rare or not often distinguished as 

 such. The extracellular haustoria usually consist of a network of short, thin- 

 walled hyphae growing over a large portion of the outer surface of the wall of 

 each algal cell. Where these are present the food must pass through the wall of 

 the haustorium and also through that of the alga, but in instances where the 

 haustoria become intracellular the passage is through the wall of the haustorium 

 only. The algal cells of the algal layer are numerous in vigorous thalli, but in old 

 and dying thalli few of them remain alive. The hyphae of this layer give rise to the 

 outer or upper cortex, whether a true cortex or a pseudocortex, and are also con- 

 tinuous with the hyphae of the medullary layer. The food assimilated by the 

 living algae, together with that which the hyphae may be able to take from the 

 substratum through the rhizoids, the umbilicus, or the hyphal rhizoids, serves both 

 for the production of new algal cells by division and for the growth of various 

 portions of the thallus. As the algal cells die and pass outward new ones are 

 constantly being formed toward the lower and inner portions of the algal layer, so 

 that the thallus is always possessed of an abundance of assimilative tissue. The 

 growth and division of the algal cells is of course most active in young and 

 vigorous thalli and in the younger and more active portions of older thalli. 



The Medullary Layer. The medullary layer lies below the algal layer in 

 horizontal thalli and within it in fruticose thalli. This layer consists of loosely 

 interwoven hyphae and is especially adapted to give strength by connecting the 

 layers above with those below, or those without with those within. It serves 

 also as a medium for the free passage of gases much after the manner of the 

 spongy parenchyma of a leaf. The differentiation into algal layer and medullary 

 layer is not always perfect even in the best-developed thalli, and algal cells may 

 occur in small numbers in the latter layer. The medulla is commonly the thickest 

 of all the layers of the thallus, and the section in this portion of the thallus is more 

 constantly transparent or hyaline than in either of the cortical layers. As the 

 hyphae of this layer serve for giving strength, the walls are scarcely ever percep- 



