62 



DISEASES OF TREES 



when their germ-tubes may either break through the lateral 

 walls or push out through the open apex of the sporangium 

 (Fig. 17, c). In either case the germ-tubes creep about for 

 some time on the epidermis of the host-plant, after which they 

 force their way into the interior, and especially at those places 



where the lateral walls of 

 the epidermal cells are 

 situated (Fig. 17, b, d}. 

 Less frequently the germ- 

 tubes reach the interior by 

 first traversing an epi- 

 dermal cell (Fig. 17, e). 

 Under favourable circum- 

 stances the development 

 of the parasite may have 

 progressed so rapidly in 

 the plant that three or four 

 days after infection new 

 sporangiophores may 

 make their appearance. 



The sporangia, and the 

 swarm-cells that form in 

 them, serve to spread the 

 disease during the months 

 of May, June, and the 

 earlier part of July. They 

 either fall directly on to 

 neighbouring plants, or 

 are carried by the wind. 

 Their distribution is great- 

 ly assisted by animals 

 as, for instance, by mice 



(in the seed-beds) and game, but most of all by man. The death 

 of all plants along a path is the result of the sporangia and 

 swarm-cells adhering to the trousers or coats of passers-by, and 

 afterwards dropping off farther along the path. 



From what has been said, the favouring influence of rain, shade, 

 c., is sufficiently evident. In dense seed-beds the hyphae grow 

 directly from one plant to another, and this offers a simple 



"IG. 17. The surface of the stem of a seedling 

 beech. At a b zoospores are seen which 

 germinate and send their germ -tubes into 

 the interior at a point where the common 

 wall of two epidermal cells abuts on the sur- 

 face ; c, a sporangium whose zoospores have 

 germinated, df, in its interior ; at e a germ- 

 tube has grown directly into an epidermal 

 cell ; at a germ-tube has reappeared on 

 the surface. 



