xiil] "DAMPING OFF" OF SEEDLING-TREES. 277 



simply boring through the cell-walls (Fig. 43). This 

 process of boring through the cell-walls is due to the 

 action of a solvent substance excreted by the growing 



Fig. 43. — Portion of a cotyledon of the beech, infested w ith Phytophthora ojnnivora : 

 the piece is shown partly in vertical section. The mycelium, spreading between 

 the cells, puts forth aerial hyph<E, which bore between the cells of the epidermis, 

 i, and d, or emerge from the stomata, a, and form conidia at their apices : the 

 various stages of development are shown. On other hyphae, between the cells of 

 the interior, the oospores are formed in oogonia, e and/! (Highly magnified.) 



tip of the hypha : the protoplasm secretes a ferment, 

 which passes out, and enables the tip to corrode or 

 dissolve away the substance of the cell-walls, It 13 



