xiir.] "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 \v ith Phytophthora otnnivora : 

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

 the cells, puts forth aerial hyphe, which bore between the cells of the epidermis, 

 b, 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, thi oospores are farmed 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 is 



