2 22 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



even under the most favorable conditions, that our knowl- 

 edge of lichens, if it is based on hand sections and cultures 

 only, must be incomplete. Hand sections are unsatisfactory 

 at the best because they are so thick that important details 

 are hidden by excess of material. Yet investigations of 

 lichens which are based only on examinations of dead 

 herbarium material, or even of freshly fixed, imbedded 

 and sectioned specimens, are subject to the same danger of 

 one-sidedness which sometimes threatens the work of those 

 who know plants and animals only as embalmed tissues. 



It is deplorable that we are still utterly unable to obtain, 

 and to keep healthily alive, thin sections of living material. 

 As Fischer (1895) and others have repeatedly emphasized, a 

 technique which is based on the employment of poisonous 

 acids and salts is one which can be implicitly trusted only 

 when their effects on the living protoplasm are exactly 

 known. The danger of being misled by the study of fixed 

 material sectioned by microtome after imbedding is, how- 

 ever, minimized by examination of living material, both sec- 

 tioned and in culture, as a control. When the question 

 before the student can be answered in part at least by deter- 

 mining the relative position of cell-walls, nuclei, etc., the 

 matter is still simpler. Such is in great measure the case 

 with the questions which are under discussion in this paper. 

 From microtome sections of lichens it is possible to obtain 

 clearer views on the structural relations of hyphae and 

 gonidia than can be had otherwise, and from these data, 

 supplemented by culture experiments, conclusions regard- 

 ing their physiological relations may safely be drawn. 



As reported elsewhere (Peirce, 1898(7), the material was 

 prepared as follows: freshly collected specimens of R. 

 reticulata, SphcB7'ophorus globife?'HS (L.) D.C., and an 

 Usnea, after being thoroughly wet, were kept in a moist 

 chamber near a window for twenty-four hours. This 

 allowed the gonidia to resume their photosynthetic activ- 

 ities in the light, and the hyphse to become reasonably 

 plump, if they had become dried out of doors. 



