CHAPTER I 

 INTRODUCTION 



The Fungi are parasitic or saprophytic Thallophyta entirely destitute of 

 chlorophyll, and possessing in the very large majority of cases a vegetative 

 portion, the mycelium, made up of filaments or hyphae. The group is a very 

 ancient one, the earliest known undoubted fungi occurring among the remains 

 of Rhynia and Hornia in the Old Red Sandstone of the Muir of Rhynie, 

 Aberdeenshire. This material consists of aseptate hyphae and vesicles which 

 doubtless served the purpose of reproduction (frontispiece) 1 . 



Fungal hyphae may be non-septate and coenocytic, or they may under- 

 go transverse septation, in which case their constituent cells are either uni- 

 nucleate or multinucleate. Any division other than transverse is extremely 

 rare; it occurs, for example, in the development of certain multicellular 

 (muriform) spores (fig. I ), and in the initiation of the perithecium in Strickeria 

 and of the pycnidium in Pleospora and Phoma*. 



As a rule the hyphae are richly branched ; they elongate by apical growth 

 and usually spread loosely through the substratum ; in certain cases, especially 

 in relation to the fructifications of the higher forms, they become woven 

 into a dense mass which gives in section the appearance of a tissue, and 

 is therefore described as pseudoparenchymatous; when fructifications are 

 embedded in such a mass it is termed a stroma; a similar weft of hyphae 

 sometimes give rise to root-like strands of which the best example is the so- 

 called rhizomorph of Armillaria mellea, or to a compact resting body or 

 sclerotium the outer cells of which are modified to form a thick-walled rind, 

 protecting the vegetative mycelium against desiccation. 



Frequent anastomoses take place between hyphae, either by means of 

 short branches forming loops, bridges or H-pieces, or by means of so-called 

 clamp-connections which join adjacent cells; such arrangements facilitate 

 the passage of food and may, in certain cases, become sufficiently numerous 

 to form a net-work. 



The mycelium begins its development as a germ-tube put out from one 

 of the numerous types of fungal spore. Where the spore wall is very thin 

 the wall of the germ-tube may be continuous with it (zoospores), but in the 

 majority of cases the wall of the germ-tube is continuous only with the 



1 Kidston, R. and Lang, W. H. On Old Red Sandstone Plants showing Structure from the Rhynie 

 Chert Bed, Aberdeenshire, Trans. Roy. Soc. Ed. 1921. 



2 Kempton, F. E. Origin and Development of the Pycnidium, Bot. Gaz. 1919, Ixviii, p. 233. 



