232 CYSTOPUS 



be studied. The parts affected by the Fungus, most commonly 

 situated in the region of the inflorescence, are swollen and con- 

 torted (Fig. 123, A), and exhibit a white surface which looks as 

 though it had been whitewashed. Such enlargement, or hyper- 

 trophy, is a frequent symptom of fungal attack, and is an outcome 

 of the abnormal development of the diseased tissue, whose cells 

 undergo increase in size with, or without, division. A longitudinal 

 section through such a blister (best stained with eosin) shows the 

 hyphae of the parasite ramifying in all directions within the 

 intercellular spaces and middle lamellae of the host (Fig. 123, B, /?.). 

 Here and there, however, small club-shaped branchlets (the 

 haustoria, 5.) will be observed penetrating into the actual cell- 

 cavities, and by this means the Fungus absorbs food-material 

 elaborated by the host. 



Near the surface of the stem the hyphae are more densely 

 packed, and their almost parallel branches form a pile-like felt 

 (the hymenium, Fig. 123, C) which ruptures the overlying epi- 

 dermis and causes the white appearance above mentioned. The 

 slightly swollen ends of the hyphae of the hymenium exhibit 

 various stages of constriction, resulting in the gradual formation 

 of chains of spherical structures called gonidia (Sp.), the oldest 

 of which is farthest away from the point of origin. As the short 

 fragile stalks connecting the gonidia with one another get broken 

 across, the latter are removed by the wind, and sometimes travel 

 many yards before reaching the ground. 



When rain or heavy dew causes a sufficient accumulation of 

 moisture, the contents of the gonidia divide into several parts, 

 which are liberated as minute colourless zoospores (Fig. 123, G), 

 swimming by means of a pair of cilia. Many doubtless perish 

 before reaching a suitable host, but should they encounter 

 seedlings of a Cruciferous plant, they come to rest on the surface, 

 secrete a membrane, and elongate into a short hypha which 

 penetrates into the interior by way of a stoma. The stimulus 

 directing the movement of the zoospore towards the host-plant 

 is probably a chemotactic one (p. 222), whilst the growth of the 

 hypha into the interior affords an example of positive chemo- 

 tropism. For some weeks further development of the Fungus 

 consists in the ramification and gradual spread of the hyphae 

 through the tissues of the host, until a sufficiently large haustorial 



