70 DISEASES OF CULTIVATED PLANTS 



plants usually continue to grow for some time before they 

 become attached by discs formed on the rootlets to the roots 

 of grass and other wild plants. In some species only those 

 rootlets that come in contact with the root of a host-plant 

 form discs, the remaining rootlets absorbing a certain amount 

 of food from the soil like ordinary plants, although root-hairs 

 are not in all cases present. For this reason this group of 

 plants has been called semi-parasites. 



Bartsia alpina (L.) possesses three distinct methods of 

 obtaining food. Certain of the rootlets are furnished with 

 root-hairs, and absorb food from the ground in the ordinary 

 manner of green plants. Other rootlets not provided with 

 root-hairs wander in the soil until they come in contact with 

 the root of a host-plant, when an attachment-disc is formed, 

 and the root is tapped for food. During the autumn small 

 brown bulbils are formed on the roots, consisting of fleshy, 

 overlapping scales, cavernous inside like the scales in the 

 toothwort. These scale-leaves are in reality traps for minute 

 animals, protozoa, etc., which serve the plant for food. 

 During the following season aerial flowering stems originate 

 from these bulbils. 



Dr. A. Fraysse has recently been dealing with the biology 

 of parasitic Phanerogams, and summarises his results gener- 

 ally as follows. Plants most readily attacked by such para- 

 sites as Lathraea, Euphrasia, etc., which attach themselves 

 by suckers, are those containing nodules of bacteria, micor- 

 hiza, tubercles, etc. Suckers are sometimes pericyclic, 

 sometimes endodermic in origin, and appear to represent 

 modified roots. The invasion of the parasite usually stimu- 

 lates the host to the formation of a cambium zone or a layer 

 of cork, for the purpose of cutting off the infected area. 

 Mucilage or gum is also often produced at the point attacked. 

 Some green parasites obtain both mineral food and carbon 

 compounds from their host, as Odontites, others, as Euphrasia, 

 absorb only carbon compounds. Parasites without chloro- 

 phyll absorb the whole of their food from the host. In all 

 instances the parasite exercises a selective power, and converts 

 the absorbed food-materials by means of diastases into com- 

 pounds that it can assimilate. Carbon appears to be mainly 

 obtained from glucose, and a special diastase for the conversion 

 of starch obtained from the host-plant into this substance. 

 This glucose may be utilised at once by the parasite or again 

 transformed into starch for future use. The suckers contain 



