

1^' 



Volume 13 Number 9 



The Plant World 



A Magazine of General Botany 

 SEPTEMBER. 1910 



THE MAKING OF PARASITES. 

 By D. T. MacDougal. 



About 2,500 species of parasitic seed-plants are known, 

 which are included in two hundred genera, representing ten 

 great families. Thorough examination of the roots of the com- 

 mon plants would doubtless result in evidence that would double 

 the number of known parasites, and give a total that would in- 

 clude about five per cent of all flowering plants, which exhibit 

 dependent nutritive habits. 



Any study of parasitic plants leads to a consideration of the 

 mycorrhizal forms, or plants which form associations with fila- 

 mentous fungi in which the thread-like hyphae enwrap or pene- 

 trate the foots of the higher plants. Such partnerships generally 

 result in some advantage to the higher plant, and are followed by 

 immediate changes similar to those exhibited by species known as 

 parasites. These changes include a lack of differentiation of the 

 tissues, even in the seed and embryo; a lessened development of 

 the shoot and root, a reduction of the leaf surface and diminished 

 production of chlorophyll. Taken together, the parasitic plants 

 and those which enter into partnerships with fungi in the soil, 

 comprise nearly half of the seed-plants of the world. It is evi- 

 dent that the existence of a tendency of the kind indicated af- 

 fecting half of the higher plants, which leads toward atrophy of 

 the vegetative organs and the development of specializations 

 of structure and habit seen in associations and dependent nu- 

 trition, must have a tremendous significance for the student of 

 eyolution. 



