276 INTRODUCTION TO PLANT GEOGRAPHY [CHAP 



properties, the vast majority are of little importance and even the 

 best tend to be overshadowed nowadays by such synthetic insecticides 

 as DDT. Nevertheless some plant products continue to be used 

 very extensively and even increasingly to combat insects and other 

 vermin, and accordingly to be of great service to Man. The three 

 most important are : (i) nicotine, extracted from leaves of Tobacco 

 plants and used as a spray that is lethal to some of the worst insect 

 pests ; (2) rotenone, obtained from the roots of various tropical 

 trees or shrubs belonging to the Pea family, and long used as a fish- 

 poison as well as, latterly, in powdered form or spray to kill various 

 insect pests of crops and livestock ; and (3) pyrethrum or insect 

 flowers — likewise used as dusts or sprays that quickly paralyse 

 insects including pests afflicting Man, and obtained from the flower- 

 heads of certain members of the Daisy family that are widely cul- 

 tivated for the purpose. 



There are also such repellents, etc., as camphor, which is obtained 

 principally from the Camphor tree but is also synthesized, cedarwood 

 oil, the Mexican ' Cockroach plant ', and the Chinese ' Thunder-god 

 vine '. Most botanical insecticides are harmless to human beings 

 and other warm-blooded animals. However, red squill, obtained 

 from the bulbs of a small herb of the Mediterranean region, is an 

 important ' raticide ', having little eflFect on animals other than Rats 

 and Mice. 



Whereas the old-fashioned herbicides are usually more or less 

 caustic chemical compounds (' weed-killers ') that kill off vegetation 

 indiscriminately, investigation of plant growth hormones in recent 

 decades has led to the discovery and use of ' hormonal ' herbicides 

 that are highly selective in their action in killing some plants while 

 leaving others, and animals, unharmed. Outstanding in this respect 

 is 2,4-D, which at suitable concentrations kills most dicotyledonous 

 plants without aflFecting most monocotyledonous ones. Thus it 

 forms an eflfective lawn or wheat-field spray, killing most of the 

 (dicotyledonous) weeds without injuring the Grass or grain crop. 

 Plant growth substances are now synthesized in quantity and are 

 used in various horticultural practices such as the promotion of 

 rooting in cuttings. 



Environmental and Ecological 



In spite of the increasing ease and eflFectiveness of transport in 

 the modern world, the availability of this or that plant product in 



