Thallophyta and Bryophyta 153 



present, to infect plants to obtain diseased tissues. Preserved tissues 

 can be purchased from the supply houses. The most satisfactory 

 killing fluids are FAA and FJ^-bichloride of mercury (Chap. 3) . 

 Chromic acid seems to become fixed in the gelatinous bacterial slime 

 and interferes with clear staining. Process the corn stem or leaf as 

 desCTibed in the section dealing with vegetative organs of seed plants 

 (Chap. 13) . Iron hematoxylin gi\cs a brilliant differentiation of 

 the bacteria. The xylem may be lightly stained with safranin or 

 gentian violet, but the slime between the bacteria must be thoroughly 

 destained (Fig. 14.1) . 



The above methods are satisfactory for the preparations of cucimi- 

 ber stems infected with the wilt organism, Enoinia tracheiphila, and 

 succulent leaves and twigs of apple or pear infected with E. arnylovora, 

 the fire blight organism. 



MYXOMYCETES 



The slime molds are customarily studied from living cultures of 

 the slimy plasmodium and from dried specimens of the fructification. 

 These spore cases are exceedingly delicate and beautiful objects. 

 Sporangia that are nearly mature can be mounted into permanent 

 slides. Transfer directly into 95% alcohol for 10 min. Pass through 

 three grades of anhydrous alcohol-dioxan at 10-min. intervals, then 

 into pure dioxan, and mount in thin dioxan-balsam. A similar butyl 

 alcohol series may also be used. The nuclei are exceedingly small, 

 and microtoming and staining are tasks for the experienced cytologist. 



PHYCOMYCETES 



The saprophytic members of this group should be studied in 

 culture whenever possible, and the use of prepared slides should be 

 discouraged. Stages that are of short duration or difficult to obtain 

 can be preserved, for either bulk material or permanent slides. The 

 representatives of this group presented below are in common use for 

 teaching, and the process for each plant has been thoroughly tested 

 and may be regarded as type processes applicable for similar subjects. 

 Strict taxonomic sequence is not maintained in the following 

 discussion; organisms that are processed by similar technitjues may be 

 discussed simultaneously. 



Zygomycetes.— T\\t order Mucorales contains the best-known 

 members of this group. Species of Rhizopus and Miicor are easily 

 grown in culture and studied to best advantage from whole mounts. 

 Developing and mature zygospores can be preserved by cutting out 



