DURATION OF WEEDS 577 



aphis is often present on docks and sow-thistle before it attacks 

 the bean crop. 



Weeds also serve as hosts for destructive parasitic fungi. Rust, 

 smut and mildew live upon various grasses, such as couch, York- 

 shire fog, and brome-grasses, and are transferred from these to 

 useful crops. 



f. Many weeds, e.g. meadow-saffron and cowbane or water- 

 hemlock, are poisonous to stock, while others, such as ramsons, 

 if consumed by cattle, communicate the offensive odour and 

 taste of garlic to milk, butter and cheese. 



3. Duration of Weeds. — The natural length of life which weeds 

 possess is of importance, and although, as explained elsewhere, 

 there is not always a hard and fast line of separation between 

 annuals, biennials and perennials, the classification of weeds into 

 these three groups is useful. The annuals complete their life from 

 seed to seed in one single growing-period. Germinating in spring 

 the seeds produce young plants which feed and grow, and ultimately 

 sometime during the summer or autumn give rise to flowers and 

 seeds. By the time, or soon after, the seeds are ripe the plants 

 die, and all that is left of them in winter is their offspring in the 

 form of seeds. 



Although each individual annual weed possesses a short length 

 of life, namely, one year or less, it usually has extraordinary 

 power of seed production. A single poppy plant frequently 

 bears more than twenty flowers, and each of these may produce 

 two or three hundred seeds. Similar enormous increase is met 

 with in groundsel, sow-thistle, campion, charlock, and practically 

 all annuals. The total number of plants which a single specimen 

 of these weeds is responsible for is often several hundred, and it 

 is, therefore, readily understood how rapidly annuals overrun the 

 ground. 



The root-system of annuals is not often very deep, but it 

 branches out in all directions near the surface of the soil ; the 

 shoots are usually furnished with many leaves, and developing 



2 o 



