72 



{Mycocaliciuni, Pyrenula) it may mature as a fungus ; otherwise it 

 perishes. 



Sufficient has been said to make it clear that the protothallus can 

 have no definite morphological or physiological significance, and for 

 that reason no great importance should be ascribed to it. There is no 

 doubt that further study of this structure will clear up various phe- 

 nomena yet unexplained or misinterpreted. 



2. DEVELOPMENT OF THE THALLUS. 



Two types or modes of growth may be observed in the thallus ; 

 (a), horizontal ; (), vertical ; the former is met with in crustaceous 

 and foliose lichens ; the latter in fruticose forms. 



(a) Horizontal Growth. 



The increase of the thallus includes apical growth (marginal 

 increase), as well as intercalary growth, that is, new cells are formed 

 and existing cells increase in thickness and length ; as a rule, new 

 cells are formed at the apical areas, though new septa may be formed 

 at any point of the hypha, especially toward the peripheral portions 

 of the thallus. 



In the lower crustaceous lichens the growing hyphae extend 

 radially, parallel with the surface of the substratum. There is no 

 evident distinction into apical areas, that is, marginal growth is uni- 

 form in certain forms. All of the hyphae, however, do not extend 

 parallel with the substratum, since even in the lowest crustaceous 

 types the rhizoidal hyphae extend vertically downward into the 

 substratum. The stimulus which causes this downward growth (posi- 

 tive geotropism) is not understood, at least it has not been determined 

 experimentally. The functional significance of the rhizoidal hyphae 

 is very evident and has already been explained. 



As soon as the thallus acquires any considerable thickness the 

 hyphae of the thallus proper no longer continue exactly parallel to 

 the substratum. Certain filaments extend downward and are contin- 

 uous with the rhizoidal ones; some remain nearly horizontal (plagio- 

 tropic), while the upper have a tendency to become negatively 

 geotropic. A vertical longitudinal section of such a thallus would 

 show that the growing elements describe an orthogonal trajectory. 

 The hyphal filaments turn upward and downward more and more as 

 we proceed from the apex, so that they finally become vertical to the 

 median plane. 



