82 DISEASES OF FIELD & GARDEN CROPS. [CH. 



The upper surface of a turnip leaf is shown at A, and 

 the lower surface at B, and it will be seen at once that 

 the spawn threads within the leaf are of too great a 

 diameter to emerge through the little lancet-shaped 

 orifices of the stomata. When the fungus threads from 

 within approach the little openings of transpiration in 

 their growth outwards, they slightly contract in dia- 

 meter, form a stop or joint, and then, instead of emerging 

 through the stomata with a pointed end, they present 

 a chisel edge to the mouth of the pore, which exactly 

 suits the shape of the little lanceolate opening. This 

 mode of emergence is shown, enlarged 1000 diameters, 

 on the left hand lower illustration of Fig. 30. The stems 

 of the fruiting threads are therefore not truly cylindrical, 

 but, when seen in section, present a flattened oval form as 

 illustrated. As the stem now gradually grows upwards 

 it usually twists round once upon itself. This twisting 

 habit is slightly retained by all the numerous branches and 

 branchlets of the fungus. The upper part of each stem of 

 Peronospora parasitica, Pers., is very much branched and 

 rebranched, and each ^ittle branchlet carries a compara- 

 tively large ovate, almost globular spore, as shown, enlarged 

 1000 diameters, in Fig. 30. The spores or conidia 

 usually germinate by bursting at the side, and the pro- 

 truded vital material or germ tube has the power of 

 piercing the cuticle of cruciferous plants. Although repre- 

 sented in the drawing as growing in an upright fashion, 

 the real growth of every Peronospora is of course down- 

 wards from the under surface of the leaf. Now, if the 

 illustrative drawing is turned upside down to present the 

 Peronospora, in a really natural manner, the resemblance 

 of the fungus to a minute bunch of grapes is a striking 

 one. From this resemblance the genus now termed 

 was for many years known as Botrytis from 

 a bunch, in reference to the resemblance to a 

 bunch of grapes. 



No zoospores have been detected in this species, but, 



