MICROSCOPICAL STRUCTURE OF LICHENS. 165 



in colour, and belongs to tlie CMorophyceae ; it is common to a 

 large number of foliose and fruticose licbens (PI. 4, fig. 1), 



There is no need for cutting sections during the preliminary 

 examination of the constituents of a lichen thallus, for it is sufficient 

 if one places a small portion upon a glass slip, and pounds it while 

 there with the flat blade of a pocket-knife. The alga and the 

 fungus will be separated, and owing to the minute size of the algal 

 cells they will be uninjured, and can be examined as separate 

 objects. A 1/6-in. or 1/8-in. objective will be required for 

 this work. 



Before satisfactory sections can be cut, it is necessary to fix the 

 material, and for this purpose a weak chromo-acetic fixing medium 

 is used, allowing the material to remain in the solution for at 

 least twenty-four hours. After the fixation, a thorough washing 

 is given, and this is continued for from two to three hours. It 

 has not been found that fixing by using successive strength of 

 alcohol produces equally good results. Fixed material is passed 

 into paraffin wax in the usual way, and sections are cut with a 

 Cambridge " rocker " microtome. 



For staining the sections, Haidenhain's iron-alum-haematoxylin 

 is the most generally useful ; Bonney's triple stain, methyl- violet- 

 pyronin and orange G. acetone, differentiates remarkably well (1). 

 Cyanin and erythrocin have been recommended for this work, 

 but the differentiation is not altogether satisfactory. 



After staining, all sections were formerly mounted in Canada 

 balsam, but some are now put up in glycerine, for by this method 

 the cell wall is often rendered remarkably clear.* 



A transverse section of the thallus of Cladonia digitata exhibits 

 a series of parallel layers, the uppermost of the series being the 

 upper cortical layer. The next in order is the gonidial layer, which 

 is followed by the medulla and the lower cortical layer (PI. 4, 



* I am glad of this opportunity for expressing my indebtedness 

 to Mr. J. H. Pledge for the help he has given me by making photo- 

 micrographs from the preparations that are being used to illustrate 

 the latter part of my communication. By means of them I am able 

 to explain more clearly the conclusions that I have arrived at after 

 the examination of a considerable number of species of the different 

 genera of lichens. Most of the photomicrographs are X 1,000 upon 

 the lantern slide. 



