DISCOVERY 



47 



Weeds 



By George C. Gough, A.R.C.Sc, H.Sc. 



What is a weed ? The first dictionary I take up 

 says " The general name of any plant that is either 

 useless or troublesome ; a plant such as grows where it 

 is not wanted and is either of no use to man or is 

 injurious to crops." A simpler definition would be 

 one on the lines of a definition of dirt — matter in the 

 wrong place. A weed is a plant in the wrong place. 



It is well known that a weed of one countr\- may be 

 the exotic of another. An instance of this occurred 

 some 3'ears ago when a gardener in England sent seeds 

 of a lovely greenhouse plant in his possession to a friend 

 in the West Indies, thinking it would grow out of doors 

 there. The friend planted the seed and anxiously 

 waited for the plant to appear, but to his disgust found 

 it was one of their common weeds. 



Why are weeds unwanted ? Not necessarily because 

 they are useless, for I know one fruit-plantation where 

 dandelions were so plentiful that their rootstocks were 

 dug up, dried, and sold for adulterating chicory, or as 

 a substitute for it. Nor because they are always small 

 and insignificant, for some are large, shapely, and 

 ornamental. 



There are many reasons, some apparent and some 

 only brought to notice by scientific research. As 

 regards the former, it is obvious that a weed takes up 

 space which should be occupied by the crop, and thus 

 robs the crop. Every plant, if it is to come to perfec- 

 tion, requires light, air, moisture, and food, but weeds 

 struggle with them for all these. A broad-leaved weed 

 frequently gets ahead of a cultivated plant and shades 

 it, with the result that, in the absence of sufficient light, 

 the latter cannot develop the green colouring-matter, 

 or chlorophyll, without which the ordinary plant is 

 unable to grow. The chlorophyll bodies, minute as 

 they are, are able to build up complex compounds, like 

 sugar, from simpler materials onlj' under the action of 

 sunlight, and therefore shaded plants cannot normally 

 thrive. They become yellow and sickly. WTieat, 

 for instance, although a vigorous plant, cannot compete 

 with weeds. At Rothamsted, the famous experimental 

 station, a wheat crop was once left unharvested to sow 

 itself without further cultivation. In three j-ears the 

 wheat had entirely disappeared in the wilderness of 

 grass and weeds which had sprung up. 



Plants require moisture, and in taking this up by the 

 root-hairs,' they also take up their food, consisting of 

 salts dissolved in the soil-water. Weeds compete with 

 them for this, and rob them of their due. In such cases 



' Tiny microscopic hairs very near the tip o{ the rootlets, 

 fresh ones being constantly formed as the rootlet grows. 



nearly evervthing depends on which gets the best start, 

 and other things being equal, the plant which gets the 

 start soon crowds out the other. It sometimes seems 

 as if it is the weed that always scores in this respect. 



Plants require air, not only because they breathe like 

 animals by taking in oxygen, but also because the 

 carbon, which is the basis of all organic substances, is 

 obtained by them from the carbon dioxide in air and 

 worked up, as stated above, into sugar and starch, and 

 even more complex compounds. The air with carbon 

 dioxide is taken into all parts of the plant through tiny 

 mouths, or" stomata," which are largely automatic in 

 their action. They are especially thickly placed on 

 the under-surface of leaves where they are not so liable 

 to be choked by dust as those on the upper surface of 

 the leaf. As plants are commonly grown for com- 

 mercial purposes as close together as is consistent with 

 health, weeds, if allowed to grow naturally take up 

 space and rob the crop of air. 



Weeds may be even more definitely harmful, as when 

 they are parasitic ■ on a crop. Perhaps the commonest 

 instance of this is in the case of dodder, which, although 

 it will prey on other plants, is frequently found on 

 clover. The dodder is a small flowering plant which 

 entwines itself around its victims and into which it 

 sendssuckers by which it extracts nourishment for itself. 

 It is practically colourless and has no chlorophyll, so 

 that it must get its complex food materials from other 

 plants instead of manufacturing them for itself. It 

 has flowers, and produces seeds which are harvested 

 with the clover seed. If the clover seed is not cleaned, 

 and the dodder separated, the latter is sown with it and 

 its life-history is repeated on the fresh clover plant. 

 In this instance the dodder gets its water and salts 

 from the ground as it is rooted, but in the case of the 

 mistletoe, the parasite gets all its water and salts from 

 the tree on which it grows. Here, however, it manu- 

 factures some of its complex compounds by means of 

 its light green leaves, and it is termed semi-parasitic. 

 Some climbing weeds are injurious because they drag 

 down the plant ; such are bryony and the bindweed, or 

 convolvulus. 



A less obvious danger from weeds, and because less 

 obvious the more dangerous, is the fact that weeds 

 become hosts for parasites of cultivated crops. The 

 farmer, by means of rotation, changes the crops from 

 field to field year by year, so that one crop does not 

 succeed the same or a similar crop. Thus potatoes are 

 followed by wheat or oats, and these by clover and so 

 on. A pest, therefore, which lives on potatoes has 

 little opportunity of living if no potatoes are planted 

 for three or four years, and it gradually dies out. If, 

 however, weeds which are related to the potato, such 

 - A parasite is either an animal or plant which lives in or on 

 another animal or plant. 



