SAPROPHYTISM AND SYMBIOSIS 



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milk, and the bacteria of hay infusions (figs. 14-17). Among the com- 

 moner saprophytic fungi are the molds (e.g. Penicillium and Mucor, 

 fig. 1078), the yeasts (figs. 168-173), an d most fleshy fungi (figs. 197-198). 

 Saprophytic fungi and bacteria occur wherever there is dead organic 

 matter, particularly in humus, the processes of decay being associated 

 with these organisms. 



The vegetative body of fungi, the mycelium, is composed of delicate 

 threads, the hyphae (fig. 1078), which penetrate the substratum in all 

 directions, often giving it a characteristic color, as in white or yellow 

 dead wood. In Lycoperdon, the hyphae form rootlike strands of different 



FIG. 1078. A diagrammatic representation of a common mold (Mucor), showing 

 branching rhizoid-like hyphae which get food from the substratum ; note also the globular 

 sporangia borne on stalks, the sporophores; considerably magnified. From COULTER 

 (Part I). 



sizes. Hyphae, like root hairs, are filamentous plasmatic structures with 

 permeable thin walls, but as might be expected from their heterotrophic 

 relations, they appear to surpass root hairs as organs of absorption, prob- 

 ably because of their greater power of penetration and because of the 

 higher concentration of their cell sap. Some hyphae penetrate dead bark 

 and wood mechanically, chiefly through cracks and wounds, while others 

 secrete enzyms, which increase the solubility and digestibility of many 

 of the materials in the substratum. Fungi secrete organic acids much 

 more actively than do root hairs, and thus are more potent factors in 

 disintegration. But it is probably the concentrated cell sap that is chiefly 

 responsible for the absorptive efficiency of fungi, since this makes pos- 

 sible not only a wider range of habitats (as jelly and sirup, in which root 

 hairs cannot develop), but also the absorption of a greater number of 



