146 THE AMERICAN MONTHLY [May 



at the joints, and are capable of reproducing the plant; 

 the vine and rose mildews seldom get beyond this stage. 

 In other cases, however, spherical bodies, called concep- 

 tacles. are found amongst the mycelium ; these usually 

 have appendages which are very various in form ; they 

 also contain asci,or sacs, each ascus enclosing the spores. 

 The appendages are often very curious and beautiful. In 

 the mildew of the garden pea they are long flexuous 

 threads. In Phyllactina guttata, found on the hazel, ash, 

 etc., they are needle-shaped with a bulbous base. In the 

 mildew of the sycamore and maple (uncinula), they are 

 divided at the extremity and recurved, resembling the 

 sign for the constellation Aries in astronomy ; in that of 

 the poplar and willow they are curved into a spiral. Mi- 

 crosphseria grossulariaj, common on gooseberry leaves, has 

 the appendages divided dichotomously at the tips and so 

 on. The mouldy appearance is readily visible to the 

 naked eye, and the conceptacles, if present, can be seen 

 with a pocket lens. In mounting, one or two should be 

 crushed so that the number of asci in the conceptacles, 

 ^•and the number of spores in each ascus can be ascertain- 

 ed. A fragment of a leaf, with conceptacles in situ, makes 

 a pretty opaque object, while some must be detached and 

 examined by transmitted light and a somewhat higher 

 power to make out the details as to appendages, etc. A 

 few forms referred to this group are saprophytes, grow- 

 ing on dead straw, pa*per, etc. 



In the Peronosporeae we have a third and very differ- 

 ent group of parasitic fungi, although somewhat resem- 

 bling those described in the last section. Like them the 

 mycelium forms a delicate bloom on the leaves, usually 

 on the underside, but it also ramifies extensively in the 

 parenchymatous tissue, the fertile hyphse finding their 

 way out through the stomata. They then usually be- 

 come somewhat branched, each branchlet bearing at its 

 tip a simple ovoid or rounded conidium. This conidium 



