362 FUNGI 



go on producing other acrospore-bearing thalli. In a third type acro- 

 spores are ahvays formed (so far as experience goes) on the primary 

 thallus produced by the germinating ascospore. This type de Bary 

 subdivides into two as follows : {a) the primary thallus arising from 

 the ascospore is only a promycele bearing sporids^ which in turn 

 produce a definite thallus, which either bears a sporocarp at once, or 

 develops as sub-type {b). There is here then a necessary intervention of 

 acrospores (sporids) before the sporocarp is again formed, ib) The 

 primary thallus arising from the ascospore is a definite one, which ulti- 

 mately bears the sporocarp, it is true, in all cases of complete develop- 

 ment, but not until acrospores have been formed on it. The sub-type 

 to some extent suggests the first type, since the acrospores are not 

 morphologically a necessary intervention, though their appearance is 

 invariable. The thallus produced by the germinating acrospore 

 resembles the primary one in all respects. The development in this 

 type may stop short with the formation of acrospores, and this is often 

 repeated in succeeding generations. 



The spores of such intervening states are invariably acrospores, and 

 they appear either singly or on hymenia on the free surface of the 

 thallus ; or they are produced in pymids, bodies resembling peritheces. 

 The spores so produced are termed stylospores, or better, pycnospores^ 

 as de Bary proposes. A species may produce only one of these kinds 

 of acrospore, e.g. Erysiphe ; or under favourable circumstances more 

 than one kind, e.g. Pleospora. 



I. Erysiphe.^:. — The mycele of the Erysipheas infests the surface 

 of green living plants, through the epiderm of which it sends dow^n 

 haustoria into the tissues beneath. The mycele is dehcate and cob-w^eb 

 like in appearance and consists of branching septate hyphae, and is 

 secured, so to speak, to the host-plant by means of the haustoria. In 

 the course of the branching of these hyphae they frequently cross each 

 other, and at such points of contact the sporocarp is formed. If Podo- 

 sphjEra (Kze.), which has only a single ascus, be taken for the sake of 

 simplicity, its development may be described as taking place in this 

 fashion. From one of these crossing hyph^, at the point of contact 

 there springs an oval cell, the carpogone, which is separated by a 

 transverse wall from the hypha. From the other hypha, likewise at the 

 point of contact, there springs also a cell, longer and thinner than the 

 other, which is similarly cut off by a septum. It overtops the carpogone 

 to which it adheres, and the upper portion, which is slightly bent over 

 the carpogone, is farther cut off from the lower by a transverse wall, 

 The upper portion is the antherid and the lower its stalk. From the 

 hyphae at the base there now grow up a number of tubes, which envelop 



