75 



lateral branches are also frequently formed, especially in the algal 

 area. 



Intercalary growth is slow in all parts of the thallus, with the ex- 

 ception of the apical portion ; it is continuous, although the thallus soon 

 obtains its full thickness ; the oldest cells (dermal, rhizoidal) are 

 continually worn away and replaced by new ones formed more in- 

 ternally. This process varies with the atmospheric conditions, but it 

 continues as long as the plant lives. 



Without exception, new septa are cut off transversely to the long 

 axis of the hyphal filament. In general, it is true that the secondary 

 changes affecting the longitudinal walls also affect the transverse 

 septa. It is also reasonable to suppose that these secondary changes 

 are simple and limited in accordance with the secondary changes in 

 other lowly organized plants. The mechanical adaptations will be 

 more fully discussed in another chapter. 



II. GROWTH OF THE APOTHEC1A 



The apothecia, which resemble the spore-bearing structures of 

 certain Ascomycetes, belong to and take their origin from the hyphal 

 portion of the lichen. In stratified thalli the apothecium begins 

 to develop beneath the algal zone ; in the so-called homoemerous 

 lichens at some point in the interior of the thallus. Its first appear- 

 ance is noticeable as a rounded mass of irregularly interwoven 

 hyphae ; from this hyphal mass numerous vertical branches arise and 

 constitute the beginnings of the paraphyses. The original hyphal net- 

 work which forms the, hypothecium increases in width much more rap- 

 idly than in thickness. The lateral proliferation does not, however, 

 extend horizontally, but in a direction outward and upward, so that 

 the hypothecium becomes concave above ; and, as the hypothecium 

 increases in width, new paraphyses are formed parallel to those al- 

 ready existing. While growth in the apothecium does not cease until 

 it is fully matured, it decreases from the periphery of the hypothe- 

 cium toward the middle ; new paraphyses are occasionally pushed up 

 between the others, especially near the periphery. 



The spore-sacs begin to form very early in the development of 

 the paraphyses ; a terminal cell of a hypha enlarges and gradually 

 pushes its way upward between the paraphyses ; it can readily be 

 distinguished from these structures by its greater size, the richness of 

 its plasmic contents, and by the fact that it is always single-celled, 



