20 



DIVISION I. — GENERAL MORPHOLOGY. 



mycelial filament according to the species is either not altered at all at the point of 

 origin of a haustorium, or is at most only slightly enlarged there ; or it has a flat nearly 

 semicircular protuberance the height of which is at most equal to its own diameter ; or 

 it has a protuberance in the form of a bluntly lobed disk scarcely exceeding the breadth 

 of the filament, which is pressed closely down on the epidermis, and appears on both 

 sides or only on one side beyond the flank of the filament. These lobed attachment- 

 disks were first discovered by Zanardini in Erysiphe Tuckcri. 



The thick mycelial tubes of the Peronosporeae. which are usually without 

 transverse walls and spread among the cells inside living plants, often clinging close 

 to the outer surface of their walls, send haustoria into the cells, which have very 

 different forms in the different species. In Cystopus (Fig. 8 A), Peronospora nivea, 



P. pygmaea, P. densa and others, 

 they are like those of the Erysipheae 

 but much smaller, and they usually 

 or perhaps in all cases only make 

 a deep indentation in the walls of 

 the cells ; in P. parasitica they are 

 lobately branched tubes, and their 

 vesicular club-shaped branches often 

 quite fill the cells of the host ; in 

 most of the pleuroblastic Perono- 

 sporeae (Fig. 8 B) they are slender 

 filiform lateral branches of the in- 

 tercellular filaments with many 

 curved and winding ramifications 

 in the interior of the cells. In Phy- 

 tophthora infestans, which inhabits 

 the potato, branches of the myce- 

 lium which in this case scarcely 

 deserve a separate name force their 

 way at various spots, especially in 

 the sprouting tubers, into the cells 

 of the host. 

 The intercellular mycelium of the Uredineae has a variety of haustoria formed like 

 those of the Peronosporeae, especially the pleuroblastic species, and with them should be 

 mentioned also the winding intracellular mycelial branches of the Ustilagineae. 



The haustoria of Piptocephalis, Syncephalis, and Mortierella are very different 

 from those which we have hitherto been considering. Piptocephalis Frcseniana is 

 parasitic on the larger Mucorini, and its mycelium, like that of its hosts, con- 

 sists of tubes without transverse walls. If a growing filament of the mycelium 

 of the parasite comes in contact with a mucor-tube, either with its apex or with 

 its lateral wall, it spreads out slightly at the point of contact and thus attaches itself 

 firmly as with a cupping-glass to the Mucor. A tuft of filiform radiating branching 

 processes of such extreme delicacy that nothing is known of their minute structure 

 now shoot forth from the middle of the surface of attachment into the cell of the host. 

 The length of these suction-filaments is about equal to the diameter of the mucor-tube 

 (see section XLIII). Van Tieghcm and Le Monnier describe similar arrangements 

 in Mortierella and Syncephalis, only in these genera the tubes which enter the cells of 

 the host are not so different from those of the rest of the mycelium. 



An allied case, though with important points of difference, is that of the clusters of 

 haustoria in Chaetocladium Jonesii, a form which is usually parasitic on species 

 of Mucor like the genera just described, and resembles them in structure. The tubes 

 of this Fungus, both of its mycelium which spreads in the substratum and of the part of 

 its thallus which rises above it, become firmly attached at the point of contact to the 



FIG. 8. Mycelial tubes m creeping about in the intercellular spaces with 

 their haustoria penetrating into the cells z—z ; A of Cystopus candidus, 

 from the pith of Lcpidium sativum, B of Peronospora catotheca from the 

 pith of Asperula odorata. Magn. 390 times. 



