PARASITISM 193 



and asci arising from the oogone of Pyronema, along with the various 

 hyphae of vegetative origin, all of which form the apothecium, while 

 only the ascospores are the reproductive cells). 



3. In the Phycomyceteae, except the Mucorales and the Entomoph- 

 thorales and allied forms, uniflagellate or biflagellate zoospores are gen- 

 erally produced as a means of asexual reproduction, while in these orders 

 the asexual reproductive structures are clearly only modifications of zoo- 

 sporangia. In the Higher Fungi the asexual reproductive cells are conidia, 

 which are separable single cells (sometimes several-celled structures) of 

 the mycelium with no indication of ever having had any homology with 

 zoosporangia. 



4. The aquatic Phycomyceteae and the terrestrial order Perono- 

 sporales have walls composed of cellulose (or carbohydrates closely related 

 to cellulose). Cellulose is lacking in the Higher Fungi, or when present 

 has been shown by Thomas (1928, 1930) to play only a subordinate part. 

 In these the basis of the wall is chitin surrounded or mixed with carbo- 

 hydrates and more or less fatty substances. Hopkins (1929) reports that 

 neither chitin nor cellulose is present in several species of the class Asco- 

 myceteae and of the class Basidiomyceteae, although other species of 

 both classes tested by him showed chitin but not cellulose to be present, 

 as is claimed by von Wettstein (1921) to be probably universal in these 

 two classes. Senft (1916) reports that the ascogenous hyphae of Chryso- 

 thrix nolitangere Mont., a lichen, respond to the standard cellulose tests. 

 The same is true, according to Honey (1936), of the disjunctors in the 

 chains of conidia of Monilinia. Malengon (1931) has shown that the 

 epispore of the spores of some Agaricaceae, e.g., species of Russula and 

 Lactarius, is blued by iodine reagents. He believes the epispore to contain 

 a mixture of cellulose and callose. This was first observed by Rolland in 

 1887. Klihner (1936) finds a similar reaction in the outer layer of the 

 internal hyphae of the pileus and stipe of some species of Mycena. The 

 Mucorales and Entomophthorales seem to occupy an intermediate posi- 

 tion in that they have chitin in their cell-walls and rarely have true 

 cellulose, although other carbohydrates are present in the wall. 



Parasitism 



The Higher Fungi are with few exceptions terrestial or epiphytic, a 

 very few species are truly aquatic, some being parasitic upon marine 

 algae, but they possess none of the characteristics of a primary aquatic 

 habit such as zoospores, etc. One order, the Laboulbeniales, consists 

 exclusively of insect-infesting parasites. The remainder are mostly either 

 saprophytes, or parasites upon plants (exceptionally upon animals). All 

 degrees of parasitism are found, varying from destructive parasitism in 

 which the tissues of the host are killed even before the invading hyphae 



