Thallophyta and Bryophyta 159 



preserved fairly well by FAA, but Craf I followed by careful 

 embedding yields superior results. For the most critical cytological 

 requirements, use the Flemming modifications as described in 

 research papers. Many stain combinations give excellent results for 

 class material, safranin-fast green is particularly good, but iron 

 hematoxylin is by far the best as a nuclear stain. 



P. coronata, the crown rust, is probably second to wheat rust in 

 importance. The uredinia and telia on Avena and other grasses and 

 the pycnia and aecia on Rhamnus (buckthorn) are treated like wheat 



rust. 



Two common species of Gymnosporangium have the telial stage 

 on J II ui penis, producing woody galls of stem tissue in which the 

 mycelium is perennial. The younger galls are soft enough to be 

 sectioned in paraffin. Divide into wedge-shaped pieces, kill in FAA, 

 and process in butyl alcohol. The pycnia and aecia of Gymnospornn- 

 giiirn juniperi-virginianae occur on Pyrus, and those of G. globosuui 

 on Crataegus. Treat like the aecial stage of wheat rust. 



Melampsora is very common on willows and poplars. The bright 

 yellow uredinia, which may entirely cover the leaf, are handled like 

 other leaf rusts. The coal-tar dyes do not seem to be so selective for 

 nuclei as iron hematoxylin. The telial stage on the old leaves is a 

 difficult problem, the host cells become very brittle in paraffin, and 

 the nuclear staining is selective only with iron hematoxylin. 



A great diversity of host tissues in which rusts are found 

 necessitates more or less specific adjustment of the killing fluid for 

 each problem. The foregoing recommendations are based on success- 

 ful preparations and will serve as a guide for other problems in this 



group. 



Tretnellales.-The order is characterized by the small, gelatinous 

 fructifications. Septation of the basidium differs in the several families, 

 and some authors regard as orders some of the families incorporated 

 here. The delicate fruit bodies must be collected in an absolutely 

 fresh condition or the time spent in processing them is wasted. The 

 portion near the substratum is of no interest; remove the substratum 

 completely, and kill the entire or subdivided fruit body in weak 

 chrome-acetic, or in Craf I. Exercise extreme care during dehydration 

 and embedding. The best stain is iron hematoxylin, with safranin-fast 

 green as second choice. 



A garicales.— The primary consideration in the processing of this 

 group is to maintain intact the more or less exposed, delicate basidia 

 and especially the exceedingly fragile sterigmata on which the 



