FUNGI. INTRODUCTORY 393 



has often been put forward that the septate filamentous Red Algae 

 gave rise to some of the septate Fungi ; but these may have arisen 

 independently from a more primitive source, the Protista, i.e., primitive 

 organisms not definitely distinguished as plants or animals. 



The lower organisms, and especially those of aquatic life, live habitually 

 in close juxtaposition. As a rule any large Seaweed or submerged fresh-water 

 plant bears innumerable smaller organisms attached to its surface. Sometimes 

 they penetrate into its tissues. Some by preference frequent certain hosts. 

 Thus Polysiphonia fastigiata is regularly borne on the Brown Tangle Asco- 

 phyllum nodosum, and its filaments extend deeply into its tissues. The proof 

 of actual parasitism is here incomplete, though the regularity of occurrence 

 arouses suspicion. But there can be no doubt of the physiological dependence 

 in Harveyella mirabilis, which grows as a colourless parasite penetrating 

 the tissue of Rhodomela, one of the Red Algae (Fig. 291). By structure and 

 propagative organs the parasite is clearly another Red Alga, which acts like 

 a true Fungus. Among the Green Algae, Coleochaete, one of the Chaetophor- 

 ales, grows habitually on the surface of submerged plants. The allied 

 Cephaleuros virescens, though still green, penetrates the tissues of the leaves 

 of Camellia. Other similar cases might be quoted from allied septate Algae. 

 Again, the green Siphonaceous Phyllcsiphon lives habitually in the intercellular 

 spaces of the leaf of Arisarum, causing discoloured patches. Such examples, 

 which might be greatly extended, show how juxtaposition may give opportunity 

 for parasitic encroachment. They are seen in modern living forms referable 

 to recognised groups of Algae. They suggest how in the past fungal parasitism 

 may have arisen. They also prove that the " Fungal " condition may be 

 arising now, as in the past, and along a plurality of evolutionary lines. 



The Fungi are very various in habit, and in form. The most 

 familiar types are the large Mushrooms and Toadstools, — or " seats 

 of death," so called in allusion to the poisonous character of some 

 species. Many of these grow on decaying humus, and like the Common 

 Mushroom are saprophytes. Others are parasites, like the large shelf- 

 fungi (Polyporus), which grow out from the trunks of trees, and 

 are the cause of the perishing of the heart-wood in hollow timber ; 

 or like the Honey- Agaric (Armillaria mellea), which kills forest trees 

 by attacking their soft and nutritious cambium (Fig. 292). But 

 apart from these there are multitudinous smaller Fungi, such as 

 the parasitic Mildews and saprophytic Moulds, while the unicellular 

 Yeasts show the simplest structure of them all. However complicated 

 and various their structure may be, it is based upon the simple or 

 branched filament, or hypha. The whole system of such hyphae is 

 called a mycelium. Such filaments may grow singly, as in the Moulds 

 and Mildews, or they may be massed together so as to form the 

 complex bodies of the larger Fungi. When closely appressed the 

 septate filaments may seem to form a definite tissue ; but it is in 



