18 



two stages of the fungus can be observed, viz., the vegetative or the 

 mycelial stage (Fig. 2) and the reproductive or the Conidal stage (Fig. 

 3). The first is represented by the mycelium which creeps on the epider- 

 mis and sends down here and there slender cellular prolongations which 

 penetrate the cell wall and swell up within the cell, forming clasping, 

 sucker-like organs called haustoria. These permit it to live at the expense 

 of the host plant. On the mycelial threads throughout their entire length 

 arise other threads composed of egg-shaped spores arranged in the form of 

 beads. Such spores, 6 to 8 in number, are called conidia and form the 

 conidal stage of the fungus (Fig. 3). As soon as the fungus matures 

 the conidia are detached and are carried by the wind to favorable plots 

 that the plant itself, or other neighboring plants, procures for them. If 

 the spore finds there conditions of humidity slightly favorable it takes 

 longer to develop. On the contrary, if the humidity be favorable the 

 condidium germinates by sending out two short germ tubes into which 

 passes the contents of the spore (Fig. 2). The first duty of the spore is 

 to fix itself to the epidermis. After Wolf, the end of one of the tubes 



Fig. 5— Eryaiphe graminis.— (A) a bundle of score sacs or asci; 

 (B) an ascus with its contained spores. 



sends out a prolongation which pierces the cell wall to form an attachment, 

 while the other germ tube develops mycelial threads like those described 

 above. Towards the end of the vegetative period of the host plant, which 

 is hastened by the development of the disease, the production of condidia 

 becomes more rare ; while on the contrary, the contiguous threads of the 

 mycelium in those parts where they are best fixed by haustoria, fuse and 

 form branches. These send out other branches which in their turn come to- 

 gether and fuse. From this confused mass of threads are formed rounded, 

 brownish bodies, immersed in a mass of mycelial threads and presenting a 

 characteristic roughness. These are the perithecia which can be likened 

 to fruits containing seeds (Fig. 4). In this state this "fruit" serves to 

 preserve the species against the severe conditions of winter. It detaches 

 itself from the leaf, falls on the ground whence it is carried by the wind. 

 A transverse section of the perithecium shows 8 to 15 asci which are 



