68 DISEASES OF CULTIVATED PLANTS 



If the nutating stem touches a host-plant it at once twines 

 round it and soon sends suckers into the tissues which hence- 

 forth supply it with food. Owing to increase in the length of 

 the stem of the host-plant, the dodder is soon carried up 

 away from the ground. During damp weather a seedling 

 dodder remains alive from two to three weeks without obtain- 

 ing food from a host-plant, the upper or stem portion simply 

 feeding upon the contents of the thickened root portion. 

 When this is consumed the seedling perishes, the entire 

 absence of chlorophyll preventing the plant from utilising 

 inorganic food. 



The toothwort (Lathraea squamaria^ L.) is entirely desti- 

 tute of chlorophyll, and obtains a portion of its food by being 

 parasitic on the roots of various kinds of broad-leaved trees, 

 and another portion from minute animal organisms which are 

 captured by its modified leaves. The entire vegetative portion 

 of the plant remains underground, the flowering branches 

 alone appearing above ground. When a toothwort seed 

 germinates, slender rootlets are produced which extend through 

 the humus until they come in contact with a slender root of 

 a suitable host-plant, when a disc is formed, from the centre 

 of which a slender sucker penetrates to the wood of the host- 

 plant. When a certain number of attachments have been 

 made and a supply of food assured, underground branches 

 covered with thick, fleshy, colourless scale-like leaves are 

 produced. The plant is a perennial, and the formation of 

 underground branches continues from year to year, until a 

 dense mass of vegetation is developed which may cover a 

 square yard or more in the loose humus or soil. The fleshy 

 leaves are furnished with a tortuous cavity in their interior, 

 which communicates by a narrow channel with the exterior. 

 This cavity is lined with absorbing glands, which extract 

 the juices of minute animal organisms that enter the cavity. 

 New roots producing attachment-discs are developed each 

 year. 



The mistletoe illustrates yet another phase of parasitism. 

 Having green branches and leaves it can do a certain amount 

 of work for itself in the way of securing and preparing food, 

 that is, it can take in carbonic dioxide from the atmosphere, 

 and this along with cell-sap obtained from the plant upon 

 which it is parasitic, by the aid of its chlorophyll, it converts into 

 starch and other requisite food. The mistletoe occurs on 

 many different kinds of trees, and is sometimes met with on 



