INJURIES INDUCED BY PLANTS 63 



explanation for the total destruction of all the plants at certain 

 parts of the bed. 



As the result of the sexual act the oospores originate in the 

 tissues of the host-plant in the following manner. In the inter- 

 cellular spaces of the leaf-parenchyma of the beech, globular 

 swellings appear at the apex of numerous short hyphal branches, 

 and become oogonia ; while smaller so-called antheridia originate 

 in like manner either at the apex of special hyphae, or on the basal 

 portion of the stalk of the oogonia. In each case a transverse 

 septum delimits the organ from its stalk (Fig. 15, c c). The 

 antheridium having very early laid itself against the outer wall 

 of the oogonium, and the most of the protoplasm of the latter 

 having become aggregated to form an oosphere, the antheridium 

 next develops a short process, the fertilizing-tube, which it pushes 

 into the interior of the female organ as far as the oosphere. A 

 part of the contents of the antheridium is then transferred to fer-j 

 tilize the oosphere, which is thereby converted into an oospore. 



In the roots of conifer seedlings oospores are formed not only 

 in the cortical parenchyma but also in the interior of the tracheides, 

 when, in consequence of the restricted space, they frequently 

 assume an elongated form. 



The oospores reach the ground in the decomposing parts of 

 plants, and there they may remain capable of germinating for at 

 least four years. Some soil taken from a diseased beech seed- 

 bed in 1875, having been distributed in water and poured 

 upon a bed of seedling beeches, caused disease and death of the 

 germinating plants not only in 1876 but also in 1878, and even 

 in 1879. 



The practical measures at our command for combating the 

 disease follow from what has been said. In order to guard 

 against the outbreak of an epidemic, we must avoid sowing seeds 

 on ground where the disease has once proved destructive, though 

 we may cultivate transplants on it. The oospores that remain 

 in the ground will prove destructive only to seedlings. Should 

 the disease appear in a seed-bed, all contrivances for producing 

 artificial shading must be removed, as they prevent the rapid 

 evaporation of water from the surface of the cotyledons. All 

 dead and visibly diseased plants should be removed. If a 

 number stand close together, the distribution of the sporangia and 



