268 



METHODS IN PLANT HISTOLOGY 



easily recognized, normal plants having the prostrate habit, while in- 

 fected plants become erect and the internodes become greatly elon- 

 gated. Aecia growing on Arisaema triphyllum (Jack-in-the-pulpit) 

 are also easy to cut. The Aecium on Hepatica has large nuclei and 



affords particularly good views of the in- 

 tercalary cells, and the origin of the bi- 

 nucleate stage. 



The Chicago chromo-acetic-osmic acid 

 solution is recommended for fixing, and 

 iron-haematoxylin with a faint touch of 

 orange is a satisfactory stain (Fig. 72). 



Without special treatment, it is prac- 

 tically impossible to get good sections of 

 sori of urediniospores and teliospores, on 

 account of the silica. Fix in formalin 

 (10 c.c), acetic acid (5 c.c), and 70 per 

 cent alcohol (85 c.c.) for at least 2 days, 

 preferably 1 week. Material may be left 

 in the fixing agent indefinitely. When 

 wanted for imbedding, rinse in 70 per cent 

 alcohol and treat with hydrofluoric acid 

 (10 c.c. hydrofluoric acid and 90 c.c. of 70 

 per cent alcohol) for 2 days, or even 3 or 4 

 days. Wash in 70 per cent alcohol, at least 

 3 changes, to get rid of the hydrofluoric 

 acid. Then proceed in the usual way. 



If a chromic fixing agent has been used, 

 treat with hydrofluoric acid after washing 

 in water. 



Everyone who studies the rusts should attempt to germinate the 

 urediniospores and teliospores. For this purpose the hanging drop cul- 

 ture may be employed, as described in the chapter on temporary 

 mounts (chap. v). The urediniospores germinate readily all summer 

 but in most forms teliospores will germinate only in the spring follow- 

 ing their maturity. However, the teliospores of "lepto" species, like 

 Puccinia xanthii on Xanthium canadense (cocklebur), will germinate 

 as soon as they ripen, and will serve equally well for study. If a par- 

 ticularly good specimen is secured, it may be preserved by the method 

 previously described for desmids, except that in this case it might be 



Fig. 72. — Aecium on Hepatica: 

 fixed in chromo-acetic acid with a little 

 osmic acid, and stained in safranin, 

 gentian violet, orange; from a prepa- 

 ration by Dr. Wanda Pfeiffer Vestal. 

 X950. 



