114 DISEASE OF TURNIPS. 



to one of the most minute veinlets) which go to strengthen the 

 leaf. At the top and bottom of the leaf is a cushion of jointed 

 spawn or mycelium, closely interwoven ; and from these jointed 

 threads arise numerous jointed club -like branches. Each branch 

 is terminated by an elongated barrel-shaped spore, densely 

 filled with protoplasm and furnished with a cell-wall. In the 

 Bradford Abbas specimens these spores measure -00125 of an inch 

 in length, and this agrees well with the size orginally given by 

 Mr. Berkeley in the Gardeners' Chronicle, viz., from '0012" to 

 0013". When Mr. Berkeley published this plant in the Annals 

 of Natural History, he changed the dimensions to -0015", and 

 this is repeated in Dr. Cooke's Handbook. A glance 'at the illustra- 

 tion will show the number of spores that are visible on the 576th 

 part of a square inch as seen edgeways ; 8 or 10 times as many 

 would be really present on the cubical piece of leaf-blade here 

 illustrated. How many spores, then, must there be on one leaf, 

 on all the plants of one field, or dispersed over the hundreds of 

 acres near Sherborne ? How, too, can the plants grow with 

 such a cushion of spawn enveloping the foliage all over ? 



The spores germinate very readily; they have only to be 

 dusted on to an uninfected leaf or on to a piece of clean glass, 

 and kept under a bell-glass in moist air. An opening speedily 

 appears at the edge of one of the ends of the barrel -like spores, 

 and through this opening a thread of spawn emerges, which 

 rapidly reproduces the parent Oidium. A germinating spore is 

 shown in fig. 74, F, a new inmature club is seen at o. The spore 

 is on a piece of Turnip-leaf, one of the stomata, or mouths, being 

 visible at c. 



The above is only a short part of the life history of Oidium 

 Balsamii : whence it comes, where it goes, what other form it 

 takes, on what plant it may then live, and how it hibernates, no 

 one at present knows. From the Gardeners' Chronicle for Septem- 

 ber 25th, 1880. 



