STRUCTURE AND FUNCTIONS 21 



The greater number of fimgi are very small. In 

 many cases they are so small that they cannot be seen 

 without the aid of a microscope. Some plants consist of 

 a single cell (Saccharomyces or yeast, and Synchytrium) 

 while others consist of long thread-like growths com- 

 posed of one or many cells and known as mycelia. In 

 some species of fungi the mycelium spreads over the 

 surface of the plant (mildew), or substances from which 

 they derive their food, while other species send their 

 mycelia into the substances and ramify throughout 

 the substratum (bread mould). The mycelia of the 

 fleshy fungi penetrate the substances on which they live, 

 but at some time in their life-history the accumulated 

 growth is concentrated at a definite point as the fleshy 

 structure with which we are familiar. 



In structure the fungi resemble the algae, the group 

 of plants to which they are the most closely related. 

 The absence of chlorophyll is the most conspicuous and 

 most important character and at once sets them apart 

 from all other plants. Although they can and no doubt 

 do absorb gases and water, and dissolve salts to some 

 extent, they cannot utilise the sunlight to elaborate 

 these raw materials into food substances for their own 

 use, but are dependent on other organisms in which the 

 raw materials have already been elaborated. 



The bread mould (Rhizopus nigricans) (Figs. 11 

 and 12) is a most excellent example of a fungus, and one 

 with which we are all familiar. It consists of a mass 

 of thread-like filaments (mycelium), which extend over 

 the surface of, and through the bread. Each thread 

 is in reality a tubular structure filled with protoplasm. 

 It acts upon the bread in such a manner as to make 

 it soluble, and then takes this soluble food material 

 in through the cell wall and makes it a part of itself, 

 thus enabling it to grow and extend farther and farther, 

 and to produce its fruit or spores (see page 24). 



While the bread mould may be looked upon as a 

 typical fungus, other fungi may radically differ from 

 it in certain characters. Some fungi are much smaller 



