FOREST WEEDS. 391 



i. The matted roots of many weeds increase the difficulty 

 of reproduction of the forest, as in the case of heather-, bilberry, 

 and couch-grass. 



ii. The removal of valuable mineral matter from the soil, 

 which thus becomes impoverished, for instance, of potassium 

 phosphate in grass-seeds. 



iii. Mechanical injury to young forest plants by birch, 

 aspen, sallow, grasses, etc., owing to deprivation of light, heat, 

 air, dew, or rain. Lightdemanding plants are thus soon 

 killed, and even shadebearing species cannot long withstand 

 such injuries. 



iv. Injuries by smothering or constricting plants, as, in 

 summer, by honey-suckle, clematis, convolvulus, wild hops, 

 etc. In winter, by being pressed down on the plants by 

 snow ; bracken, tall grasses, etc. 



v. Retention of excessive moisture in the surf ace- soil during 

 wet weather, and formation of swamps ; consequent increase 

 of damage by frosts : all peat-plants, and especially Sphagnum, 

 increase the swampiness of the soil. 



vi. A dense growth of grass or weeds may prevent dew or 

 light rains, which merely dry off the surface of the weeds, from 

 penetrating the soil, and is thus very hurtful to plantations 

 and sowings during droughts. This may be easily proved by 

 digging up a sod and examining the soil beneath it. Besides 

 excluding moisture from the soil, the grasses, etc., draw up 

 the soil-moisture from below and transpire it into the air, 

 so that the denser the growth of grass, the drier the soil 

 becomes. 



vii. Certain plants produce a sour or dry humus which is 

 unsuitable for most forest trees' : this is the case with heather, 

 reeds, and other sour grasses, sedges, etc. 



viii. Shelter is afforded by grass and herbage to mice and 

 insects. 



ix. Weeds and especially heather, increase danger from 

 forest fires. 



x. Parasitic plants, other than fungi, that remove sap 

 from forest plants. They may be classified as follows : 



. Parasites germinating and growing in the living tissues 

 of their hosts (mistletoe and LorantJms). 



