WEEDS 835 
312. How weeds injure the farm and garden.! Although 
some weeds are of use as food for man or the lower animals, 
and a few have medicinal properties, their presence in the farm 
or garden is on the whole most harmful in the following ways: 
1. Weeds take 
moisture needed 
by useful plants. 
2. Weeds rob 
the soil of val- 
uable salts, such 
as nitrates and 
potash compounds, 
and it is probable 
that they may add 
secretions that are 
injurious. 
3. Weeds shade 
other plants, thus 
weakening them 
by hindering pho- 
tosynthesis. 
4. A few para- 
sitic weeds, like 
the clover dodder 
(fig. 243), rob 
their hosts of 
plant food. 
5. Some weeds 
harbor _ parasitic 
fungi or insects 
(suchas the potato 
beetle) which are 
injurious to use- 
ful plants. 
Fie. 242. The common ragweed (Ambrosia 
artemisiaefolia) 
The group of flower clusters at the left and the leaf at 
the right are considerably reduced, and the central plant 
is much reduced. The slender stem is characteristic of 
individuals grown in the shade; plants grown in the 
sunlight are much shorter and very robust 
1See also Bergen and Caldwell, Practical Botany. Ginn and Company, 
Boston. 
