80 



111 other species of the genus the spores develop in a different 

 manner. The spores fall into a drop of water-rain or dew, 

 where their protoplasm breaks up into a number of distinct 

 pieces, which escape from the cell and are al)le to swim about 

 by means of two cilia with which they are furnished. After 

 swimming about for a time they settle down on the plant, send 

 out a germ tube into the epidermis and begin to develop in its 

 tissues in the manner described above. 



This formation of motile spores is of common occurrence in 

 the Fungus of the potato disease, and during moist, damp weather 

 increases considerably its power of extending rapidly over large 

 areas. 



The manner in whicli the first tube, developed from the spore, 

 penetrates the epidermis varies in the different species. It is of 

 course necessary for the young Fungus to obtain food material 

 as soon as possible, as it contains very little, if any, reserve 

 food material in itself, and its first care, therefore, is to obtain a 

 safe footing in the host plant. In the case of the potato disease 

 fungus, the motile spores, as soon as they come to rest on the 

 epidermis of the potato plant, surround themselves witli a delicate 

 cell-wall and immediately afterwards put forth a tube which 

 penetrates at once into the host plant by piercing the cell-wall, 

 it thus obtains food at once, and is able to pass into the inter- 

 cellular spaces beneath the epidermis where it begins to 

 ramify and form a mycelium. In other cases the spore sends 

 out germ tubes which pass through the stomata of the plant 

 directly into the intercellular spaces, and from these gradually 

 pushes its way into all parts of the plant. 



Sexual Method of Reproduction. 



There are two kinds of sexual organs, male, antheridia, and 

 female, oogonia. The oogonia are formed as swellings on the 

 hyphfe. They may be either terminal or intercalary. In the 

 former case the oogonium begins to form as a slight expansion of 

 the end of a filament, this gradually expands into a globular sac 

 of considerable size, which becomes filled with protoplasm and 

 nuclei obtained from the hypha. When the oogonium has reached 

 its full size, it is cut off from the hypha by a transverse wall. 

 (Fig. VI.) 



