112 DISEASE IN PLANTS. 



roots for salts and other materials. On the other 

 hand, the checking of surface roots by competition 

 with the grass has been claimed as advantageous. 

 In this connection probably the whole question of 

 the composition of the turf arises, as well as that 

 of possible cropping for hay, and manuring. 



As regards any particular weed, the cultivator 

 should learn all he can respecting its duration, 

 seeding capacity, method of dissemination, the 

 depth and spread of its root-system, and any 

 other particulars which enable him to judge when 

 and how to attack it. It is only necessary to see 

 the victory of such drought-resisting weeds as 

 Hieracium pilosella, Plantains, HypocJiaeris, on lawns 

 to realise how weeds may win in the struggle for 

 existence with the finer grasses. 



Many so-called weeds are, however, partially 

 parasitic, with their roots on the roots of others 

 e.g. Rhinanthus, Thesiuni, etc., and much damage 

 is done to meadow grasses and herbage by the 

 exhaustive tax which these semi-parasites impose. 



This is carried still further in the case of such 

 root-parasites as Orobanche^ where the host-plant 

 is burdened with the whole support of the pest, 

 because the latter, having no chlorophyll, is 

 entirely dependent on the former for all its food. 



Even ordinary climbing plants may injure 

 others by shading them, either by scrambling over 

 their branches e.g. Bramble, or twisting their 

 tendrils round the twigs e.g. Bryony, or twining 

 round them -e.g. Woodbine, Convolvulus, etc. The 

 principal direct injury is in these cases owing to 



