MYCELIUM AND HYPH^ PHYCOMYCETES 231 



according to the substratum upon which it occurs. The extreme 

 simplicity of the vegetative structure may well be compared 

 with that of parasitic Flowering Plants (cf. p. 179), some of 

 which encountered only in the Tropics have as a matter of 

 fact a plant-body so reduced that it resembles a mycelium. 



In some Fungi the wall of the hyphae consists of cellulose, 

 but much more commonly of a complex nitrogenous compound 

 similar to the chitin found in animals, together with other sub- 

 stances such as pectose, callose, etc. Embedded in the lining 

 layer of cytoplasm in the lower forms are numerous minute 

 nuclei, but in the septate hyphae of the higher types there are 

 usually only one or two in each cell ; neither plastids nor starch- 

 grains are ever present, but there are often small oil-drops and 

 sometimes crystalline albuminous bodies. The central vacuole 

 is prominently developed. Where abundant food-storage occurs, 

 as, for instance, in the reproductive cells, it is customary to find 

 the polysaccharide glycogen, which can be recognised by the deep 

 brown colouration assumed with iodine. In coloured hyphae, 

 such as occur in species of Peziza, etc., the pigment is generally 

 confined to the cell-wall. 



The Fungi are classified in three groups Phycomycetes, 

 Ascomycetes, and Basidiomycetes each of which has so many 

 characteristic features that it will be convenient to consider them 

 separately. ^The Phycomycetes, which are not modified to so 

 marked an extent as the other two groups, include forms which 

 usually show a well-marked sexual process, and which, in this 

 and other respects, resemble Algae such as Vaucheria. The 

 hyphae, for example, contain numerous nuclei, and often only 

 exhibit transverse walls in relation to the formation of repro- 

 ductive bodies. The group includes many common parasites, 

 such as Cystopus (the White Rust of Cruciferae, Fig. 123, A), 

 Pythium debaryanum (the cause of the " damping off " of seed- 

 lings, Fig. 124, B), Phytophthora infestans (the Potato Blight), 

 Empusa (responsible for a disease of house-flies), as well as the 

 saprophytes Mucor (the black Mould appearing on jam, bread, 

 etc.), Saprolegnia, and Achlya (the last two frequent on decaying 

 water-plants). 



Cystopus, a species of which often attacks the Shepherd's 

 Purse, furnishes a typical example*, whose life-history can easily 



