3IO PLANT LIFE. 



to give even a general description of it. Any unhealthy 

 or wounded tree is quickly attacked by Polypori, such 

 as P. squainosus^ P. betulinus, Agarici of the group 

 Pleurotus, and especially by A. melletis, and by Stereum 

 hirsutuin and S . purpureiiin. These fungi always attack 

 the cambium and phloem first. They loosen and 

 destroy the tissues between the bark and the wood. In 

 the space thus formed, the Bark-beetles, Centipedes, 

 Woodlice, and other creatures establish themselves. 

 Their eggs are laid in the most sheltered positions, 

 and their grubs cause further destruction. Sometimes, 

 however, the bark-beetles precede the fungi and attack 

 the softer tissues of scarcely injured trees. 



The medullary rays, which it will be remembered 

 extend inwards into the wood, and consist of thin- 

 walled cells with plenty of food material, are also 

 attacked, both b}^ the fungi, and by these insects. 

 After these preliminary processes, two groups of fungi 

 become of special importance. The little cups of 

 Ascomycetes (see p. 238) are found on the dead wood 

 below the loose bark. In the same position, many of 

 the Slime fungi or Myxomycetes occur. During their 

 active condition, and even in the stage in which they 

 form crawling slimy ^^ Plasmodia !' they explore the 

 medullary rays ; and the whole woody mass, and the 

 timber on which they have been at work, soon crumbles 

 into dust (see p. 236). As soon as the cork is broken 

 through, the disappearance of the log is relatively very 

 rapid. The destruction of the cork is generally the 

 result of several years' decay. Probably mosses, 

 lichens, and especially fungi, are the most important 

 agents in this destruction. Certain kinds of fungi, such 

 as Nectria Ciniiabarina and others, habitually break 

 through the cork to form their fructifications. Beetles 



