INJURIES INDUCED BY PLANTS 



149 



stem or root becomes limp. At this stage of the disease the 

 interior of the plant is nearly full of a luxuriant mycelial growth. 

 In a short time large numbers of bacteria appear in the tissues 

 and induce complete decomposition, the mycelium of the para- 

 site sharing the common fate. In plants that still appear sound 

 towards the top, the stem or roots frequently retain only the 

 epidermis and the xylem of the vascular bundles. I have 

 infected vigorous pine and spruce seedlings, which were taken 

 from dense seed-drills and planted in flower-pots, by laying 

 one or more diseased plants amongst them. When a bell- 

 glass was placed over the flower-pot all the plants were 

 diseased or dead in four days. 

 The mycelium enveloped the whole 

 plant, and the disease began, for 

 the most part in the upper portion 

 (Fig. 83, c). On the other hand, if 

 the flower-pot was left in a room 

 uncovered, infection was induced 

 only by the mycelium growing in 

 or on the surface of the soil and 

 attacking the roots or the lower 

 portion of the stem (Fig. 83, a, &). 

 All the plants succumbed in eight 

 days, with the single exception 

 of one seedling which remained 

 healthy. No results followed trials 

 at infection which I conducted in 

 the end of June on vigorous seed- 

 lings. As has already been proved in the case of other parasites, 

 it is only when the epidermis is unprovided with a cuticle that it 

 can be dissolved by the mycelium. It is known that P. omnivora 

 also is only destructive in May and June. 



Innumerable gonidia develop in dense bunches on the 

 luxuriantly branching mycelium, and especially in the stomata 

 of the diseased plants. I have represented such a mycelial 

 branch with gonidia in Fig. 86. When ripe they are more or 

 less falcate, pointed at both ends, and generally consist of six 

 cells. In germinating they usually produce two germ-tubes at 

 or near the apices, as is shown in the lower part of Fig. 86- 



FIG. 85. Epidermis of a pine 

 seedling showing a stoma. 

 Solution has taken place 

 wherever the filaments have 

 been in contact. 



