4 INTRODUCTION 



characteristic of the majority of the Phycomyceteae, produces septa to 

 set apart the reproductive organs (sporangia or gametangia) from the 

 hypha, or to fence off an injured region, or to separate an empty portion 

 of the hypha from those portions still containing protoplasm. A cellular 

 mycelium may have multinucleate cells in the younger portions but by 

 intervention of septa the older portion is transformed to cells with one 

 or two nuclei, depending upon the phase of the mycelium. An old my- 

 celium, no longer able to continue normal growth, may undergo nuclear 

 division without septum formation so that the old cells may have several 

 nuclei. In the Higher Fungi that generation of development in which the 

 cells are uninuclear is sometimes called the monocaryon phase and that 

 in which the cells are binuclear the dicaryon phase, or primary and 

 secondary mycelium, respectively. In general the growth in length and 

 the formation of new cells takes place in the terminal portion of the 

 hypha or of its branches. 



Septum formation occurs, as in most algae, by the production of a 

 circular shelf which gradually grows inward until (in the Phycomyceteae) 

 it makes a complete septum or (in the Higher Fungi) leaves a small 

 central perforation through which there is a continuous protoplasmic 

 connection from cell to cell. In Allomyces, one of the Phycomyceteae, 

 septa may be formed but these are imperfect with large openings and are 

 called pseudosepta. 



The mycelia of the majority of fungi are hyaline, especially the hyphae 

 that are embedded in the substratum and functioning as organs for ob- 

 taining nutriment. These are sometimes colored but the color is much 

 more frequent in those hyphae that are external. The pigment causing 

 this dark color is related to melanin and is largely confined to the cell 

 walls. The hyphae that bear the conidia or that protect the other types 

 of reproductive structures are especially apt to be dark-colored. 



Aside from their elongated thread-like form the hyphae may be 

 packed together tightly so that they adhere in elongated strands which 

 sometimes have a hard black external layer and creep long distances. 

 These are called rhizomorphs. In ArmiUariella mellea (Vahl) Karst., these 

 black, shoestring-like strands creep under the bark of roots and trunks 

 of trees and through the soil. Storage organs called sclerotia are frequently 

 produced by fungi. Dense masses of hyphae arise and the short cells 

 enlarge laterally until a compact pseudoparenchymatous tissue is formed 

 whose cells become polyhedral by mutual pressure. These are filled 

 with food materials and the walls may remain thin or become somewhat 

 thickened, especially the outermost layers of cells which thus form a 

 protective cortex which may be light in color or more often brown or 

 black. These sclerotia if small, as in Sclerotium rolfsii Sacc, may be trans- 

 ported easily by surface water during heavy rains. Larger sclerotia may 



