6i4 



THE POPULAR SCIENCE MONTHLY. 



is a familiar destructive fungus pest that will serve our purpose in an 

 attempt to show the preparation these low forms of plant-life make 

 for passing over hard times, including the winter season. In early- 

 summer the lower surface of affected grape-leaves is covered with a 

 minute forest of white filaments. The threads of the fungus run 

 in all directions through the tissue of the leaf, and, coming to 

 the surface, pass through the breathing-pores of the epidermis, and 

 afterward branch and bear oval spores on the tips of the filaments. 

 Fig. 2 represents a small portion of a branch of the lettuce-mildew, 

 which is in the same genus, and closely related to the mildew of 



the grape. The branch is magnified two 

 hundred times, and the spore A and its 

 attachment are enlarged five hundred di- 

 ameters. The multitudes of spores borne 

 upon the tips of the branches are for 



Fig. 2.— Branch or Lettuce-Mildew. Fio. b.— Formation oy Grape -Mildew, Winter Spore. 



the rapid propagation of the mildew. They germinate by producing 

 zoospores — that is, the contents divide into six or more oval bodies, 

 which soon rupture the spore-wall and escape, each provided with two 

 hair-like appendages termed cilia. These motile zoospores reproduce 

 the mildew in a new place upon the same or another grape-leaf. Later 

 in the season, and within the substance of the leaf or fruit, a second 

 form of spore is found. This is termed a sexual spore, and requires 

 the union of the contents of two peronospora filaments for its produc- 

 tion. One of the thread-tips becomes much enlarged, as shown in Fig. 

 3, o (on the left), which represents the female cell charged with granular 

 protoplasm. Another thread, w, arises near by, becomes club-shaped, 

 and applies itself to one side of the female sphere. The contents of 

 the male cell enter and mingle with those of the female, after which 

 the latter matures into a spore. The process of fertilization is further 

 shown in the portion of the engraving on the right, all parts being 

 magnified three hundred and fifty times. The dark central portion is 

 the spore, the contents of which become securely protected by the 

 thick, hard coverings. Unlike the small, thin-walled spores borne upon 

 the tips of the aerial branches, these large sexual spores remain through 

 the winter before germinating. The vitality of the grape-mildew is 



