THE LIVING ENVIRONMENT. in 



Various species of so-called eelvvorms Nema- 

 todes also cause gall-like swellings on young 

 roots, or they invade the grains of cereals. 



Finally, various slugs and snails cause much 

 injury by devouring young leaves and buds and 

 diminishing the assimilatory area. 



Plants as agents of disease or injury fall 

 naturally into the two main categories of flowering 

 plants (Phanerogams) and Cryptogams, among 

 which the fungi are the especially important pests. 



Beginning with weeds, we find a large class of 

 injurious agents. Weeds damage the plants we 

 value by crowding them out in the struggle for 

 existence, as already stated, and when the weed- 

 action is simply due to superfluous plants of the 

 same species, we speak of overcrowding. But it 

 must not be overlooked that the competition 

 between crowded plants of the same species- 

 where every individual is acting as a weed to the 

 others may be more dangerous than between 

 plants and weeds belonging to other species and 

 genera, because in the former case they are strug- 

 gling for the same minerals and other necessary 

 food-materials : a matter of importance in connec- 

 tion with the rotation of crops. 



The question of allowing grass to grow at 

 the foot of fruit trees, as in orchards, is a good 

 case in point. Such grass may increase the damp 

 and shade, thus favouring fungi at one season, 

 and dry up the moisture of the soil to the injury 

 of the fine superficial roots at another, as well as 

 exhaust the soil, owing to the competition of the 



