28o INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



and Cereal Rusts and Smuts are due to Fungi, which alone cause 

 many hundreds of different plant diseases, often of devastating effect 

 and vast importance. Other plant maladies may spoil amenities, an 

 example being the Dutch Elm disease which threatens the various 

 Elms that form such an important feature of the landscapes in 

 temperate regions on both sides of the North Atlantic. Some higher 

 plants and Fungi, particularly, are dangerously poisonous ; but 

 although large doses may be lethal, small ones are often beneficial 

 (as we saw when discussing drugs). Consequently such plants are 

 not unmitigated curses ; and indeed, provided they are known and 

 understood, they may be of real benefit. On the other hand, some 

 of the selective herbicides may, like disease-provoking organisms, 

 yet be turned to destructive use in biological warfare. 



The saprophytic Fungi and Bacteria that cause harmful decay, 

 putrefaction, and often loss of food and fabrics, etc., have also been 

 mentioned above ; in many circumstances there would be no organic 

 breakdown without them. At all events the beneficial ' scavenging ' 

 which these organisms accomplish is so essential as immeasurably 

 to outweight such nuisances as they perpetrate, considerable though 

 the latter may be — especially with the deterioration and spoilage 

 that are so rapid and marked in the tropics. 



Weeds are often defined as ' plants growing where they are not 

 wanted ' — which is a broader conception than we had in dealing 

 with them in the last chapter. However, the same plant that is 

 useful in one place may be obnoxious in another, in which its 

 necessary control or eradication becomes a laborious and costly 

 procedure ; in short, it has become a weed. Modern methods of 

 weed control include chemical spraying with such herbicides as 

 2,4-D or chlorates, mulching, and the biological use of ' smother 

 crops ' which combat the nuisances by competition. Other methods 

 include the introduction of diseases of the pests involved, as well 

 as such time-honoured activities as weeding, hoeing, harrowing, and 

 the like. Also effective is prevention of the growth of weeds by 

 the use of ' clean ' seed, sterilization of the bed by heat or chemicals, 

 and removal of nearby sources of infection. 



Besides being nuisances in the ways mentioned in the last chapter, 

 some weeds, such as the Ragweeds and many Grasses, have airborne 

 pollen to which people suffering from hay-fever are particularly 

 sensitive. Such plants as Poison-ivy are also a great nuisance to 

 some individuals. And finally, not only field and garden crops but 

 also forests and even waters have their weeds — in forests the often 



