45 



filament penetrate the alga ; this penetration varies greatly in de- 

 gree ; the tip of the haustorium may pass through the algal cell- 

 wall, forming a somewhat expanded filament between the wall and 

 cell-plasm. In its highest development the haustorium, after entering 

 the algal cell, develops a much-branched network which encloses 

 but does not penetrate the cell-plasm. This haustorial network is 

 made up of delicate gelatinized hyphal branches. Intracellular 

 haustoria are comparatively rare ; I have observed them most fre- 

 quently in young growing species of Cladonia, Parmelia and 

 Stereocaulon . The question as to the manner in which the haus- 

 toria gain entrance into the interior of the alga requires further study ; 

 also the question of the biologic significance of this form of union. 

 Can the liberated algal cell with its haustoria develop into a new 

 lichen? It consists of both symbionts, all that is required for the 

 formation of a new thallus ; it may, however, be that the haustorium 

 when separated from the parent filament can no longer continue to 

 grow. Algae which have intracellular haustoria have also, as a rule, 

 extracellular haustoria. The plasmic contents of the algal cell are 

 in time entirely absorbed by the haustorium, so that nothing re- 

 mains but the algal wall enclosing the hyphal network. (PL i. 

 f-5,6.) 



(d) The Medullary Layer. 



In the majority of lichens this layer is much thicker than any of 

 the others ; it usually consists almost entirely of hyphae ; rarely we 

 find distributed through it small groups of algae. The hyphal fila- 

 ments consist of branching much elongated cells and are morpho- 

 logically closely allied to ordinary fungal hyphae. The cell-walls do 

 not become gelatinized like those of the cortical and dermal layers, 

 and, like all lichen tissues, do not stain readily. Occasionally we 

 find a coloring matter deposited within or upon the cell-walls ; as a 

 rule, however, they are colorless. In the middle portion of the layer 

 the hyphae extend in a direction parallel to the main axis of growth ; 

 above and below they extend vertically to the cortical layers with 

 which they are continuous. 



Structurally the medullary layer forms a marked contrast to the 

 cortical layers. It consists of a network of very loosely interwoven 

 hyphae with large air spaces. In this respect it is analogous to 

 the spongy tissue of the foliage leaves of higher plants. Occasion- 



