68 MORPHOLOGY 



or chondroid strands of compact filaments that occur frequently in the 

 thallus of the larger fruticose lichens, and are of service in strengthening 

 the fronds. The term plectenchyma is now generally used for pseudo- 

 parenchyma. 



B. Types of Thallus 



Three factors, according to Reinke^ have been of influence in determining 

 the thalHne development. The first, and most important, is the necessity to 

 provide for the work of photosynthesis on the part of the alga. There is 

 also the building up of a tissue that should serve as a storage of reserve 

 material, essential in a plant the existence of which is prolonged far beyond 

 the natural duration of either of the component organisms; and, finally, 

 there is the need of protecting the long-lived plant as a whole though more 

 particularly the alga. 



Wallroth was the first to make a comparative study of the different 

 lichen thalli. He distinguished those lichens in which the green cells and 

 the colourless filaments are interspersed equally through the entire thallus 

 as "homoiomerous" (Fig. 2), and those in which there are distinct layers of 

 cortex, gonidia, and medulla, as "heteromerous" (Fig. i), terms which, 

 though now considered of less importance in classification, still persist 

 and are of service in describing the position of the alga with regard to the 

 general structure. A less evident definition of the different types of thallus 

 has been proposed by Zukal^ who divides them into "endogenous"' and 

 "exogenous." 



a. Endogenous Thallus. The term has been applied to a compara- 

 tively small number of homoiomerous lichens in which the alga predominates 

 in the development, and determines the form of the thallus. These algae, 

 members of the Myxophyceae, are extremely gelatinous, and the hyphae 

 grow alongside or within the gelatinous sheath. In the simpler forms the 

 vegetative structure is of the most primitive type: the alga retains its 

 original character almost unchanged, and the ascomycetous fungus grows 

 along with and beside it (Fig. 4). Such are the minutely tufted thalli of 

 Thermutis and Spilonema and the longer strands of Ephebe, in which the 

 associated Scytonema or Stigonema, filamentous blue-green algae, though 

 excited to excessive growth, scarcely lose their normal appearance, making 

 it difficult at times to recognize the lichenoid character unless the fruits also 

 are present. 



Equally primitive in most cases is the structure of the thallus associated 

 with Gloeocapsa. The resulting lichens, Pyrenopsis, Psorotichia, etc. are 

 simply gelatinous crusts of the alga with a more or less scanty intermingling 

 of fungal hyphae. 



1 Reinke 1895. 2 Zukal 1895, p. 562. 



