734 FUNGI 



Var. iv. aires on Aira caespitosa. 

 „ V. agrostidis on AgrosHs alba and A. canina. 

 „ vi. poa on J^oa compressa and P. pratensis. 



These varieties cannot be distinguished from each other 

 raorphologically ; they are, however, biologically distinct in so 

 far that the uredospores of one variety cannot infect host- 

 species different from those on which they are commonly 

 found. 



Thus, the uredospores of var. iii. cannot produce 'rust' on 

 wheat or barley, neither can the uredospores of the variety on 

 wheat produce the disease on oats or barley. 



Ex. 297. — Examine the ' rusted ' leaves of wheat in spring and ako later in 

 summer with a pocket lens. Make a sketch of the form and length of the 

 spore-beds or sori. Note the colour of the Son and the leaf-tissue round 

 them. 



Ek. 298. — Scrape oflf some of the uredospores of the above with the point of 

 a sharp pen-knife and transfer to water on a glass slide. Examine the spores 

 with a high power. Observe their form, the colour and markings on the 

 cell-walls, and the colour of contents. 



Ex. 299. — Cut transverse sections of a fresh leaf through a youngish 'rust' 

 sorus. Examine with a high power. Note and sketch the stalk or pedicels 

 of the uredospores and the mycelium in the soft tissue of the leaf. 



Ex. 300. — Brush or scrape oflF some of the loose uredospores from a leaf into 

 it drop of water on a glass slide. Place the latter on damp blotting-paper 

 under a bell-jar and examine with a low power every twelve hours until 

 germination takes place. 



Examine the germinated spores with a high power. Observe the length 

 and origin of the germ-tube. Note the colour of the contents of the 

 latter. 



Ex. 301. — Cut transverse sections of the stem or leaf-sheath through a black 

 teleutosporc sorus. Examine the section in water with a low and also with a 

 high power. Observe the length of the pedicels of the teleutospores and the 

 form, colour, and thickness of the cell-walls of the latter. 



Ex. 302. — In autumn procure straw with teleutospore-sori visible; tie in 

 a bundle and leave out of doors all the winter. In March or April cut off 

 small portions of the stem with teleutospores on them and place these pieces 

 in water in a watch^lass. Keep the whole under a bell-jar and examine 

 every twelve hours until germination takes place. A film-like mould is seen 

 with a pocket lens when germination has occurred. Mount in water on a 



