218 STRUCTURAL BOTANY 



mention, what everybody knows, that several of the 

 larger kinds are exceedingly good for food, we may point 

 out that the saprophytes, at any rate, do good service by 

 causing decay, and so ridding the world of the useless 

 remains of dead animals and plants. Masses of dead 

 material would otherwise accumulate to such an extent 

 as to interfere seriously with the life of succeeding 

 generations. Fungi and other saprophytes (notably 

 the Bacteria) bring about the decomposition of dead 

 organic matter, use the products for their own nutrition, 

 and ultimately convert its substance into simple in- 

 organic bodies (such as ammonia and carbon-dioxide), 

 thus rendering it available for the nutrition of green 

 plants, and, through them, for the support of other 



organisms. 



We will begin our illustrations of the Fungi with a 

 simple type, representing a .Family which is of special 

 scientific interest, from its evident affinity with certain 



of the Algae. 



TYPE XVIII. PYTHIUM BARYANUM 



We have chosen as our first type of Fungi a genus 

 which stands very near to the Algae, showing an un- 

 mistakable affinity with Vaucheria. The species of 

 Pythium are parasitic on seedlings, and often do great 

 havoc among them, especially if the seed-beds are kept 

 too damp, and not sufficiently exposed to air and light. 

 One of the commonest species, P. Baryanum, can be 

 obtained almost with certainty by growing Cress-seed- 

 lings under a bell-glass, and giving them an excessive 

 amount of water ; but, unfortunately, the parasite 



