brambles, willows, poplars, and other plants. After such plants 

 have occupied the ground for some time, reproduction fre- 

 quently reappears beneath their shade and again establishes 

 itself on the area, the inferior species thus appearing to have 

 prepared the way for the Spruce and Silver Fir and to have 

 made their existence once more possible. 



Although in all these cases it is obvious that certain plants 

 are in some way helped by, and are in a measure dependent 

 for their existence on, others, the exact way in which this help is 

 given is by no means always clear. 



In some cases mere shade from intense light may be the 

 beneficial factor. In other cases the shade may be beneficial on 

 account of its effect on the temperature of the plant and soil, 

 and on the quantity of available moisture in the soil and air. 

 In other cases again the benefit may consist solely in the fact 

 that the shade has been sufficient to prevent the growth and 

 development of injurious plant competitors, as would appear 

 to be often the case when shrubs prepare the way for trees by 

 killing out grasses. 



It has been often thought that plants during life con- 

 tinually excrete poisonous waste products, the accumulation 

 of which in the soil might render impossible the continued 

 existence either of the same species or of other plants and 

 that therefore some plants were able to poison the plants they 

 displaced and that others even made their own existence im- 

 possible after a certain period. Although in the case of the 

 higher plants this has not yet been proved, still it does un- 

 doubtedly hold good in the case of many fungi and bacteria. In 

 the case of a particular tree therefore occupying a given area for 

 a long period, it is possible that the accumulation of substances 

 excreted by fungi and bacteria in the soil may render impos- 

 sible the existence of those symbiotic fungi and bacteria which 

 are employed in breaking down the humus, in which case 

 the soil may become so impoverished that it can no longer 

 support a crop of the tree in question, see also the remarks on 

 page 95 in connection with the rotation of crops and also those 

 on page 207. Similarly the existence of the symbiotic fungi 

 which form the so-called mycorhizas may be prevented for 

 a time, and the natural regeneration of certain tree species 

 consequently rendered impossible. Any factor injuriously 

 affecting these symbiotic organisms, such as insufficiency of 

 oxygen or water, excess of water, too low a temperature, absence 

 of sufficient organic food materials, and so on, would obviously 

 produce the same result, 



