204 



Distant 



Symbiosis 



between 



Green and 



Non-green 



Plants. 



Decay of 

 Wood. 



ments here and there extending into the soil, and at other 

 times they cover the roots externally with a felt-like mantle. 

 Such mycelial growths are called mycorliizas. In Europe 

 it has been found that if trees like the oak and pine are grown 

 in sterile soil and are thus deprived of these mycorhizas their 

 growth is retarded and sometimes entirely prevented. The 

 precise nature of the symbiotic relation in this case is not yet 

 known, but it is probable that the host supplies the fungus 

 with carbohydrates, while the fungus helps the host to obtain 

 water, mineral salts (probably phosphates and potassium 

 chiefly) and simple nitrogenous compounds. 



181. The above are all instances 



of close symbiosis, i.e. in which two organisms live in 

 intimate connection with each other. As an example of distant 

 symbiosis we may take that existing between green and non- 

 green plants. 



We have had frequent occasion to mention above the 

 decomposing action of many bacteria and fungi which give 

 rise to various processes popularly called rotting, putrefaction, 

 fermentation, nitrification, etc., such organisms being able 

 to feed on complex organic materials and to break them down 

 into simpler substances on which the higher green plants 

 feed. By the activity of such organisms, for instance, the 

 carbon dioxide required by green plants is being continually 

 returned to the air, while nitrogenous organic substances 

 such as albuminoids, amides, and others are also broken down 

 and nitrates produced which are the chief source of nitro- 

 genous food for higher plants. Many of these decomposing 

 processes are accompanied by a liberation of energy in the 

 form of heat, and we know that the heat may be so great, 

 in a stack of rotting hay, for instance, that spontaneous com- 

 bustion is produced. 



182. Before leaving this sub- 

 ject of decay a few words will be added on the decay 

 of timber. In the first place care must be taken to 

 distinguish between the terms dead and decayed. Plant 

 tissues become dead when their elements lose their living 

 protoplasmic contents, they do not become decayed until 

 they have undergone a further process in which they become 

 disintegrated and their structure is destroyed. A piece of 

 sound, Teak heart- wood, which is really dead tissue, and which 

 is known to be the best and most durable wood for construc- 

 tion, compared with a fragment from a hollow tree which 



