FUNGI. 59 



the lower surface of the leaves, and spread rapidly through the intercellu- 

 lar spaces. 



Later on, spores of a very different kind are produced. Unlike those 

 already studied, they are formed some distance below the epidermis, and 

 in order to study them satisfactorily, the fungus must be freed from the 

 host plant. In order to do this, small pieces of the leaf should be boiled 

 for about a minute in strong caustic potash, and then treated with acetic 

 or hydrochloric acid. By this means the tissues of the leaf become so 

 soft as to be readily removed, while the fungus is but little affected. The 

 preparation should now be washed and mounted in dilute glycerine. . 



The spores (oospores) are much larger than those first formed, and 

 possess an outer coat of a dark brown color (Fig. 33, H). Each spore is 

 contained in a large cell, which arises as a swelling of one of the filaments, 

 and becomes shut off by a wall. At first (Fig. 33, F) its contents are 

 granular, and fill it completely, but later contract to form a globular mass 

 of protoplasm (G.), the germ cell or egg cell. The whole is an oogonium, 

 and differs in no essential respect from that of Vaucheria. 



Frequently a smaller cell (antheridium), arising from a neighboring 

 filament, and in close contact with the oogonium, may be detected (Fig. 

 33, F, G, cm.), and in exceptionally favorable cases a tube is to be seen 

 connecting it with the germ cell, and by means of which fertilization is 

 effected. 



After being fertilized, the germ cell secretes a wall, at first thin and 

 colorless, but later becoming thick and dark-colored on the outside, and 

 showing a division into several layers, the outermost of which is dark 

 brown, and covered with irregular reticulate markings. These spores do 

 not germinate at once, but remain over winter unchanged. 



FIG. 34. Fragment of a filament of the white rust of the shepherd's-purse, 

 showing the suckers (7i), x 300. 



It is by no means impossible that sometimes the germ cell 

 may develop into a spore without being fertilized, as is the 

 case in many of the water moulds. 



