'66 JOURNAL OF THE [July. 



■characteristic structure of the lichen. The groups became cov- 

 ered above with a very dense growth of hyphje ; below, the my- 

 celium kept pace and usually exceeded the growth above, but at 

 the sides rarely did any considerable thickening of the filaments 

 occur. So there was always room, not at the top, but at the sides 

 for the extension of the mycelium with its captured cells. This 

 new lateral extension would be woven over, almost as soon as 

 formed, by the dorsal and ventral hyphal layers. The divisions 

 and growth of the algae were very rapid in the early stages, and this 

 growth continued at the margins ; but as the hyphae, in the ex- 

 tension of the thallus, entangled them more and more in a dense 

 mass, their activity lessened and finally stopped through the crowd- 

 ing of the surrounding filaments. It was apparent that division 

 only ceased when all the space available to the algae had been 

 occupied. And so finally the form of the algae was concealed 

 by the mat of hyphae above, through whose semi-transparent 

 walls the color still shone. On the under side the thickened 

 mass shut out cells and color alike, thus presenting a whitened 

 surface. From this under side extended numerous strands of the 

 mycelium, rhizoids, into the substratum. These roots were con- 

 fined for the most part to the median and especially to the distal 

 portions of the thallus, and were of the nature of holdfasts. A 

 cross-section of the thallus (Fig. 4) now shows the hyphee so 

 closely woven together above as to render the form of the indi- 

 vidual fibrils almost indistinguishable. Beneath are the crowded 

 gonidia, forming a well-marked zone in which the original centres 

 of growth are still indicated by group-like masses of alga, though 

 this latter arrangement is often obliterated by the broadening of 

 the clusters and their consequent union. It will be noticed that 

 scattered algal cells frequently appear in the ventral medulla and 

 but very rarely in the dorsal. Thus again is it manifest that the 

 transportation of the gonidial cells attendant upon growth de- 

 pends upon their seizure by, and subsequent growth of, one of the 

 hyphae. The looseness of the subgonidial layer permits at many 

 places of such a limited growth, while above there is rarely op- 

 portunity for a fibril with its host to crowd through. Mention 

 should be made here of an exception to this rule. The gonidial 

 zone occasionally extends quite to the upper surface of the thal- 

 lus. At such places the unimpeded algae may be carried out by 



