FOREST WEEDS. 389 



capacity for being heated, porosity, etc., has more influence 

 on the growth of plants than its chemical composition, though 

 the latter has an indirect influence on the physical nature of 

 soils. 



In the strife l)etween weeds and cultivated plants, the former 

 gain ground by the easy dissemination of their seed by wind, 

 water, or birds, as well as by their superiority in the struggle 

 for light and for space for their roots. If cultivation of the 

 fields were to cease in Europe, it is certain that, in 100 years, 

 only grassland, forest, or swamp would be found, according 

 to the degree of humidity of the soil. 



/. Demands as to Liijht. 



Plants are termed lightdemanding or shadebearing accord- 

 ing to their relative demands for light, or capacity for bearing 

 shade. 



Heather is a decidedly liglitdemanding plant, bilberry 

 {Vaccinium Myrtillus, L.), a half shadebearer : the holly, 

 juniper, and Daphne are shadebearing plants, as they flourish 

 in dense woods. Ivy {Ilcdera Ileli.v, L.) and butcher's broom 

 {Ruscns acnleatus, L.) are also even more shadebearing. 



(J. Kind of Injury done. 



Weeds may overtop young trees and deprive them of light 

 and dew ; they may constrict them, as in the case of the 

 woodbine, or, like the bindweed, completely stifle them by 

 growing over them ; they may smother and bend them down 

 when pressed on them by snow, as dead bracken ; or, like the 

 peat-moss, Sphaynum, cause swamps. Some weeds, such as 

 bilberry, may form a dense mat with their roots, so that 

 natural regeneration of trees may be obstructed. 



//. Degree of Injury done. 



Weeds may be classed as very injurious, injurious, or 

 only sUyhtbj injurious. Many forest weeds are either 

 technically or indirectly useful, as will be explained in the 

 next paragraph. 



