202 



Methods in Plant Histology 



LICHENS 



The lichens are usually regarded as difficult forms. In younger 

 stages they occasion no trouble, but an old apothecium or a leathery 

 thallus often fails to cut well. By employing the gradual processes 

 already described in chap, ix, satisfactory sections should be ob- 

 tained from thalli and mature apothecia of Physcia, Usnea, Sticta, 

 Collema, Parmelia, and Peltigera. 



Cyanin and erythrosin is a very good stain for lichens. The 

 algae stain blue and the filaments of the fungus take the red. Where 

 the association of the alga and the fungus is rather loose, as in 

 Dichonema, more satisfactory mounts can be made by staining in 

 eosin, or haem-alum and eosin, and then teasing slightly with needles 

 and mounting in glycerin. 



BASIDIOMYCETES 



This is an immense group, of which the smuts, rusts, mush- 

 rooms, toadstools, puffballs, and bracket fungi are the most widely 

 known representatives. 



The Smuts (Ustilagineae) .- -The smuts are abundant on wheat, 

 oats, corn, and various other plants. 



FIG. 56. Puccinia graminis: photomicrograph of aecidium stage on barberry leaf. 

 X12. 



The smuts may be studied in the living material. The following 

 method, described by Ellis, is worth remembering: A supply of 

 smutted barley may be obtained by sowing soaked, skinned barley 

 that has been plentifully covered by Ustilago spores. In such 

 material it is easy to trace stages in the development of spores. 

 Freehand sections of ears about 12 mm. long show the mycelium and 

 spore clusters. If smutted ears be removed and kept floating on the 



