57 



t*xtend above the surface of the upper cortical layer (Plate 3, fig. 

 i). This cephalodium is, therefore, a small thallus of some other 

 lichen and has no organic connection with the plant upon which it 

 occurs. Soon after the soredium locates upon the thallus it begins to 

 grow ; the projecting hyphal branches of the cortical layer to which 

 the soredium becomes attached branch and grow in length something 

 after the manner of the algal haustoria. The cephalodium enlarges 

 in all directions, but more horizontally than vertically, which causes 

 it to become flattened ; the outer cells soon become cortical ; this cor- 

 tical tissue increases more rapidly at the upper surface, where cortical 

 projections are also formed downward for the purpose of increasing 

 the mechanical support. The cell-walls of the outermost cortical 

 cells both above and below are colored a dark brown ; they are 

 otherwise much like the cortical cells of the thallus. The medullary 

 hyphae and the algae fill the entire interior. The algae are ar- 

 ranged in chains which extend in a vertical direction ; they are some- 

 what more closely crowded toward the upper surface. 



The presence of this cephalodium has considerable influence 

 upon the underlying structures of the lichen-thallus. The cells of 

 the cortical layer of the latter become elongated and less cortical 

 in structure until in the mature cephalodium the cortical tissue has 

 become medullary. The algae of the thallus immediately below the 

 central point of the cephalodium begin to disappear quite early in 

 the development of this structure. The disappearance of the algae 

 and the cortical tissue keeps pace with the growth of the cephalo- 

 dium, so that the horizontal diameter of the cephalodium is always a 

 little greater than the diameter of the area devoid of cortex, and still a 

 little larger than the area devoid of algae. The physiological expla- 

 nation of this structural change in the thallus coincident with the 

 development of the cephalodium is a transfer of the function of as- 

 similation from the areas of the thallus covered by the cephalodia to 

 the cephalodia themselves. The haustorial hyphae of the thallus 

 adherent to the cephalodium and continuous below with the medul- 

 lary hyphae carry the food substances to and from the cephalodium. 

 Figures i, 2 and 3, Plate 3, will aid in explaining the development 

 of this form of cephalodium and its relation to the thallus. 



As Forssell has shown, there are various forms of ectotrophic 

 cephalodia. The one above described is the most common and 

 typical. Another form is commonly met with among the various 



