228 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



lichens, however, haustoria are never formed; the hyphal 

 branches investing the gonidia osmotically absorb enough 

 food to supply the whole body of the fungus. The 

 principles underlying the absorption, and the results of 

 the absorption, of food by the hyphae from the gonidial cells 

 are identical whether haustoria penetrate or hyphal branches 

 only clasp the gonidial cells. 



Microtome sections confirm the observations regarding 

 the relative sizes, the number and kinds of division, the 

 general appearance of the gonidial cells invested by and 

 free from hyphae, as reported in the foregoing chapter. 

 Both the fixed and the living specimens show that fungus 

 and alga are associated intimately enough, whether the 

 fungus merely encloses or actively penetrates, for the 

 organism absolutely unable to elaborate non-nitrogenous 

 food to be able to absorb all that it needs from the one that 

 can elaborate it. In both cases the absorption is by osmosis, 

 and it must be almost, if not quite, equally effective in both 

 cases, for the lichens which do not send hyphal branches 

 as haustoria into the gonidia appear to thrive as well as 

 those which do. 



There must in both cases be irritation of the gonidial 

 cells by the hyphae. The irritation produced by haustoria 

 must, however, be greater, and this conclusion is substan- 

 tiated by the larger number of dead gonidial cells contain- 

 ing haustoria than of those with hyphae merely attached. 

 The observation of haustoria within gonidial cells, though 

 not a step necessary to the conclusion that the fungus is 

 parasitic upon the alga, is confirmatory evidence, for it 

 proves that the fungus has the alga in its power — that it 

 obtains all its food from it, that it irritates and exhausts it 

 in proportion to the intimacy of its relation with the alga. 



In the other two forms studied — Sphcerophorus globiferus 

 and an Usnea — microtome sections show that the relation 

 between hyphae and gonidia is intimate, the hyphce closely 

 investing the gonidia. Figure 19 is from a section through 

 an old part of a Usnea, the species of which I did not 

 determine, and shows a hypha, old and thick-walled, still 



