59 



layer bulges upward more and more, which necessitates increased 

 cell-formation in the layer in order to avoid any breaks in the con- 

 tinuity of the tissue ; the algal as well as the medullary layers also 

 curve upward, but the algae decrease in number with the increase 

 in the curvature. In the fully developed superior cephalodium there 

 are only a few algae near the apex, or none ; from that point they grad- 

 ually increase in number until they reach their normal quantity at 

 the level of the algal layer of the normal thallus. Between the al- 

 gal layer of the thallus and the tissue of the cephalodium proper 

 there is a hyphal tissue devoid of algae, which corresponds in part to 

 the medullary tissue of the thallus. The remaining structure con- 

 sists of the enormously developed inferior medullary portion of the 

 thallus bearing the foreign algae, which are collected in groups 

 separated by bundles or plates of longitudinal hyphae extending 

 parallel with the algal chains ; these bundles and plates no doubt 

 serve a mechanical function. 



The inferior cephalodium corresponds to the superior in develop- 

 ment and general structure ; the algal layer of the thallus, how- 

 ever, remains unchanged ; the foreign algae are somewhat less 

 numerous, while there is an increase in hyphal tissue. 



Figures 3 and 4, Plate 4, represent the structure of both the 

 inferior and superior cephalodia ; the sections were made from 

 young undeveloped tubercles. The full grown forms are represented 

 in the diagramatic figure, 2. Figure 1, Plate 4, is a semidiagramatic 

 vertical section of a young cephalodium before it can be determined 

 whether it will become superior or inferior. 



6. THE RHIZOIDS. 



The rhizoids are the root-like structures extending from the lower 

 surface of the thallus and penetrating the substratum ; they are in 

 all cases hyphal cells more or less modified and continuous above 

 with the hyphal cells of the thallus. Structurally we may divide all 

 rhizoids into three kinds : 1. Those occurring in crustaceous lichens 

 and consisting of a hyphal network of only slightly specialized cells. 

 2. Those consisting of much elongated simple or slightly branched 

 hyphae with cell-walls firm and considerably thickened ; they 

 occur typically in the genus Mallotmm. 3. This is the highest form 

 and consists of simple or branched bundles of elongated closely 

 adnate hyphal cells whose walls are also considerably thickened and 



