Miscellaneous. 



164 



[February, 1910. 



Hook, as the botanical name of Cou- 

 gayam grass, and so appears in ' Flora, 

 British India.' I misread Mr. Lock's 

 note to me on the subject, and was only 

 able to correct my error too late for 

 insertion in the January 'Tropical Agri- 

 culturist.'' I am greatly handicapped 

 by having no books of reference of my 

 own." 



C. DRIEBERG, 



Secretary. 



Colombo, February 7, 1910. 



WEEDS. 



[Paper read before the Board of Agri- 

 culture by R. H. Lock, Acting Director, 

 Royal Botanic Gardens, on Febuary 7th, 

 1910.] 



A weed is generally defined as a plant 

 out of place. Most frequently weeds 

 are also useless plants, but useful and 

 even cultivated plants may also become 

 pests when they persist in making their 

 appearance in places where they are not 

 wanted, 



The objections to weeds amongst culti- 

 vated crops are so obvious that it is 

 scarcely necessary to allude to them. 

 The greater number of these objections 

 may be summed up in the single word 

 "competition." The weeds compete 

 with the cultivated plants for space, air, 

 and light, as well as for the water and 

 soluble constituents of the soil. The 

 removal of this competition is one of 

 the primary and most fundamental 

 operations of agriculture. The loss 

 of crops caused by allowing weeds to 

 grow freely may easily amount to 50 

 per cent, or more, and the presence of 

 weeds in the soil enormously increases 

 the labour which has to be expended 

 in tillage and cleaning operations. 



Other more or less minor disadvantages 

 are peculiar to special kinds of weeds. 

 Climbing weeds may overrun a crop 

 and bear it to the ground, or they may 

 choke the individual plants by the 

 tightness of their coils. The seeds of 

 certain weeds may contaminate the 

 crop of grain or other valuable seed, 

 and cause a marked deterioration in 

 its market price. Other weeds are 

 poisonous to stock, whilst others, again, 

 may habour insect pests and parasitic 

 fungi, which sooner or later find their 

 way to the cultivated crops. 



Many of the worst weeds of any given 

 district will be found to have been intro- 

 duced into it from some other district or 

 country. Thus, many of the corn-field 

 weeds of England have been introduced 

 at different times from Eastern Europe. 



In spite of the many excellent means of 

 distribution which Nature provides for 

 the dissemination of seeds and other 

 reproductive parts of plants, it is usually 

 found that the majority of the least 

 desirable weeds of any country have 

 been introduced through human agency, 

 either accidentally mixed with the seeds 

 of useful economic plants or other com- 

 mercial produce, or in many cases deli- 

 berately, owing to some beauty of the 

 plant having been recognized, but not 

 its harmful tendeucy to spread where it 

 was not wanted. 



Natural means of dispersal suffice, 

 however, to transport the seeds of 

 innumerable weeds over considerable 

 distances. Whenever forest land is 

 cleared the weeds of the surrounding 

 country soon begin to appear upon it, 

 and, if unchecked, may take complete 

 possession of the cleared soil until it 

 becomes impossible to grow any crop 

 without an enormous expenditure on 

 weeding. The principal non-human 

 agencies for the dispersal of seeds are 

 two : the wind and wandering animals. 

 A. great number of beautiful contri- 

 vances are to be found in Nature adapt- 

 ing seeds to travel by one or other of 

 these means of conveyance, and for a 

 description of these I must refer my 

 hearers to the work of Darwin and other 

 writers. The practical conclusion is that 

 no more land should be cleared at one 

 time than can be taken into immediate 

 cultivation. The cost of weeding begun 

 as soon as the "burn-off" is completed 

 is an insignificaut item compared with 

 the cost of eradicating weeds when they 

 are once firmly established. For the 

 seeds which come from a distance are 

 few and scattered, and of many which 

 set out upon their journey few travel 

 over long distances. The majority of 

 even the strongest winged seeds fall to 

 the ground close to their starting point. 

 Cleared jungle land is practically free of 

 the seeds of weeds, and the compara- 

 tively few casual arrivals can easily be 

 destroyed before their own seeds ripen. 

 But once the first colonists are permitted 

 to set seeds themselves, their progeny 

 springs up in constantly increasing 

 numbers until the whole available space 

 is occupied. 



The first principle in destroying weeds 

 is therefore to attack them before they 

 have had time to ripen seed. If the seeds 

 which fall at a given time all germinated 

 at once, it would be easy to extermin- 

 ate any weed which is solely seed-pro- 

 pagated in a comparatively short space 

 of time. Unfortunately the seeds do not 

 all germinate at once ; many lie 

 dormant for varying periods, so that 



