CHAPTER VI. 



PARASITIC WEEDS. 



ORDINARY green plants use the soil and the atmosphere as 

 storehouses from which they draw the raw materials of their 

 food, these being then converted by the vital processes of the 

 plants into substances which can be utilised to build up the 

 tissues. The roots, by means of fine root hairs, take up from 

 the soil water in which nitrogen (in the form of nitrates), and 

 compounds of such mineral substances as potassium, phos- 

 phorus, iron, calcium, magnesium, etc., are dissolved. This 

 dissolved raw food passes up through the woody part of the 

 plant into the leaves, where it meets with supplies of carbon 

 which are taken in from the air in the form of carbon-dioxide. 

 During the daytime complicated chemical changes take place 

 resulting ultimately in the formation of such substances as 

 sugar and starch, which are then conveyed to the various parts 

 of the plant where they are needed. This preparation of food 

 can only go on in green parts in the presence of light, so that 

 plants that do not possess green colouring matter or in which 

 leaves are absent and are not replaced by green stems are not 

 able to prepare their own food, but must obtain it ready for 

 use from some other source. Some plants of this nature 

 attach themselves in various ways to other living plants and 

 steal their food all ready prepared, thus earning the name of 

 parasitic plants. 



Parasitic plants may be divided into two classes : 



(1) Total parasites, in which green colouring matter is 

 entirely absent, and which cannot prepare any food for them- 

 selves, so that they are absolutely dependent upon other plants 

 for their food. 



(2) Partial parasites, which possess a certain amount of 

 green colouring matter which is often very poor in quality ; 

 they are able to prepare part of their food, but are dependent 

 upon other plants for most of their nutriment 



Total parasites are capable of doing great harm to the 



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