CHAPTER III 



MORPHOLOGY 



GENERAL ACCOUNT OF LICHEN STRUCTURE 

 I. ORIGIN OF LICHEN STRUCTURES 



THE two organisms, fungus and alga, that enter into the composition of the 

 lichen plant are each characterized by the simplicity of their original structure 

 in which there is little or no differentiation into tissues. The gonidia-forming 

 algae are many of them unicellular, and increase mainly by division or by 

 sporulation into daughter-cells which become rounded off and repeat the life 

 of the mother-cell; others, belonging to different genera, are filaments, 

 mostly of single cell-rows, with apical growth. The hyphal elements of the 

 lichen are derived from fungi in which the vegetative body is composed of 

 branching filaments, a character which persists in the lichen thallus. 



The union of the two symbionts has stimulated both, but more especially 

 the fungus, to new developments of vegetative form, in which the fungus, as 

 the predominant partner, provides the framework of the lichen plant-body. 

 Varied structures have been evolved in order to secure life conditions favour- 

 able to both constituents, though more especially to the alga ; and as the 

 close association of the assimilating and growing tissues is maintained, the 

 thallus thus formed is capable of indefinite increase. 



A. FORMS OF CELL-STRUCTURE 



There is no true parenchyma or cellular structure in the lichen thallus 

 such as forms the ground tissue of the higher 

 plants. The fungal hyphae are persistently fila- 

 mentous and either simple or branched. By 

 frequent and regular cell-division always at right 

 angles to the long axis and by coherent growth, 

 a pseudoparenchyma may however be built up 

 which functions either as a protective or strength- 

 ening tissue (Fig. 36). 



Lindau 1 proposed the name "plectenchyma 

 for the tangled weft of hyphae that is the prin- p . g ^ v<jrtical section of 

 cipal tissue system in fungi as well as lichens. young stage of stratose thal- 



~, . , , , , , lus (Xanthoria parietina 



1 he more elaborated pseudoparenchyma he desig- Th Fr ^ flj plectenchyma of 



nates as "paraplectenchyma," while the term cortex ;<>, medullary hyphae ; 



* c, gomdial zone, x 500 (after 



prosoplectenchyma he reserved for the fibrous Schwendener). 



1 Lindau 1899. 



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