416 SCIENCE PROGRESS 



of sunlight and air, so that anything that hinders this access is an 

 agent by which the plant is practically more or less starved. 



4. The weed seeds spoil the harvest of crop seeds by 

 rendering them impure. The admixture necessitates special 

 processes of separation and cleaning, entailing much expense. 

 In addition, the seeds of some plants — Corn Cockle for example — 

 are very poisonous, so the value of the crop for feeding purposes 

 is very much lowered, as there is danger of poisoning stock if 

 the animals are allowed access to such noxious plants. 



5. Some weeds are particularly obnoxious as they choke up 

 the threshing machine. The Wild Oat (Avciia fatuci) has a 

 twisted and bent awn and the seed is also clothed with long 

 hairs ; these seeds get into the working parts of the threshing 

 machine and cling together by their awns and hairs until they 

 block every outlet, necessitating frequent stoppages to free the 

 machinery. 



6. Weeds provide harbourage for various insect and fungus 

 pests which would either be entirely absent or at least far more 

 easily dealt with if the ground were clear of alien plants. For 

 instance, during the early summer months, when root-crops — 

 Turnips, Swedes and Mangels — are practically lacking in the 

 fields, the Turnip " fly," the Turnip-Gall Weevil and the 

 Diamond-Back Moth all thrive on Cruciferous weeds such as 

 Charlock, whereas they would die of starvation from lack of 

 food were the weeds absent. Similarly, Thistles, Dandelions 

 and Fat Hen afford cover for the Mangold Fly ; whilst the 

 Celery Fly perpetuates its existence by infesting the Cotton 

 Thistle. The Cruciferous weeds also act as hosts for such fungus 

 pests as "Finger and Toe" in Turnips, whilst the White Rot 

 of Cabbages attacks the Shepherd's Purse, which indeed acts 

 both as a host and as the initial source of infection for cultivated 

 plants. Some of the Rust Fungi which work such havoc among 

 cereal crops need two distinct hosts at different periods of their 

 life histories and in some cases it happens that the second host 

 is provided by certain species of weeds. Since the second host 

 is essential the absence of the weeds would involve the dying- 

 out of the pest. 



As stand has been made so constantly against the growth 

 and spread of weeds, practically the only plants which reach 

 any degree of significance are those provided with very efficient 



