DISEASE OF TURNIPS. 



113 



moisture, and this year the exceeding dry month of August, 

 now followed by a humid September, has accelerated the growth 

 of the Oidium to an unwonted extent : so much so, that a sports- 

 man traversing a field of these roots soon gets his trousers and 

 boots white with the myriads of shed spores. The prevalence of 

 mildew always argues a comparatively short crop of roots. 

 Professor Buckman says that there is reason to believe that this 

 onslaught, taken in connection with a wide attack of Puccinia 

 graminis on grass, is doing mischief to sheep this year. 



To the naked eye the foliage of the swedes is white on both 

 sides with the mildew : under a low power of the microscope 

 this white coating is seen to be a dense felted mass of spider- 

 web-like threads, dotted all over with uncountable thousands of 

 oblong spores. But the higher powers of the microscope are 

 required to see the exact nature of the fungus and the leaf 

 it grows upon. As the parasite has not hitherto been 

 illustrated, it is here engraved for the first time in the Gardeners' 

 Chronicle, where the fungus was originally described. Let any 

 reader of this journal get an infected leaf, and cut a piece 

 one-eighth of an inch square out of the leaf-blade. This piece 

 is far too large for a microscopist ; so, with a lancet sharper, finer, 

 and with a better temper than any razor, this small square piece 

 of turnip leaf must be cleanly cut into 24 long thin slips or rods 

 these rods, in their turn, must be again cut across each into 24 

 minute cubes, or 576 pieces, out of the eighth-of-an-inch square. 

 If one of these very minute atoms be now dexterously taken up 

 with the tip of an exceedingly small camel-hair brush, placed 

 under the microscope and examined with a good light, all the 

 points of structure of leaf and fungus may be clearly seen as in 

 fig 73. The part from A to B shows the structure of the blade of 

 a turnip leaf, as seen in transverse section ; A is the upper 

 surface, B the lower. At c, c, c, may be seen the mouths, 

 tomata, or organs of transpiration of the leaf ; at D the cells 

 of which the leaf is built up with the intercellular air passages 

 where the cells are more loosely compacted together. At E is 

 seen (cut across) one of the bundles of spiral fibres (answering 



