WEEDS-PECULIARITIES AND DISTRIBUTION 417 



means of reproduction. Reproduction may be either vegetative 

 or from seed — sexual reproduction — and whilst most plants are 

 chiefly dependent upon one of these two methods, a few of the 

 worst weeds are equally well provided with both. 



(a) Sexual Reproduction. — The weeds that occur frequently 

 enough and are sufficiently distributed to become serious pests 

 are usually notable for the large number of seeds produced by 

 a single plant and many are provided also with an efficient 

 distributing mechanism. The worst weeds in many places are 

 the various kinds of Charlock, a species of Brassica, to which 

 genus such cultivated plants as Cabbage, Turnip, Rape, 

 Mustard and Swedes belong. The mature plant bears numer- 

 ous seed pods, each containing several seeds 1 ; on this account 

 the species are among the most difficult to eradicate when once 

 they are well established. The enormous quantities of seed 

 produced are well shown by the regular armies of Charlock 

 that spring up on any spot on which a rick has stood if in 

 the previous year this weed has been garnered with the crop. 



The Curled Dock {Rumex crispus) is another weed of which 

 each individual produces many hundreds of seeds. Each seed 

 or rather each fruit is enclosed in a little capsule made of 

 three winged bracts, so that the seeds get scattered far and 

 wide by the wind when they are detached from the parent 

 plant. The same method of distribution of seed by means of 

 winged fruits is seen in the Pennycress {Thlaspi arvense), though 

 it is not so frequent among the low-growing herbaceous weed 

 plants as it is among the tall trees like Elm, Maple and Lime. 

 In the different species of Poppy each individual produces an 

 enormous number of seeds unprovided with any mechanical 

 devices but the distribution of the seeds is secured owing to 

 the fact that they are exceedingly small and light ; they are 

 enclosed in capsules which open when ripe by means of pores 

 or valves, so that in favourable weather the tiny seeds escape 

 readily and are blown hither and thither by the wind. 



The seeds of the various members of the order Compositae, 

 such as the Thistle, Dandelion, Sow Thistle and Hawkweed, 

 are distributed by a very special method. The flower head in 

 these plants is compound or " composite " in nature, being made 

 up of numerous tiny florets crowded together and surrounded 



1 Nobbe states that as many as 4,000 seeds are produced on a single plant. 

 See Long, Journ. Bd. Agric. xviii. No. 6, p. 461. 



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