220 THE BIOLOGY OF FLOWERING PLANTS 



seedlings. Rhinanthus and Euphrasia can complete their 

 development without a host, but Melampyrum sylvaticiim 

 and M. pratense do not develop fully unless they can fix 

 on the roots of neighbouring shrubs or herbs. In nature 

 the close meadow and pasture vegetation in which these 

 plants grow, extraordinarily favours their parasitic tendency. 

 The soil is full of roots and the parasites show little or no 

 specialisation ; they can prey on any neighbouring plant. 



They become attached by side roots, which remain 

 quite short and swell into tubercles. If such a root touches 

 the root of a grass, for instance, it partly envelops it, and 

 then sends into it an absorptive process in which tracheids 

 arise connecting the wood of the host with that of the 

 parasite. A single parasite attaches itself to many roots of 

 one or several hosts, belonging to one or more species. 

 Under such circumstances the parasite thrives more 

 vigorously than by itself. 



The green leaves of these plants are functional. It has 

 been shown that they form starch in light when isolated from 

 possible hosts, so that the assimilation must be quite vigorous. 

 Kostychew (1922) has shown that the intensity of assimi- 

 lation of Rhinanthus, Melampyrum, Pedicularis, Euphrasia, 

 and Bartsia is about the same as in the autotrophic Veronica 

 longifolia, and Linaria vulgaris. The root system is not 

 luxuriant, and, though certainly capable of absorbing water 

 and salts, it cannot cover the normal requirements of the 

 shoot. The same investigator found that a cut shoot of 

 one of these partial parasites absorbed water from twice 

 (Euphrasia) to ten times (Melampyrum) as rapidly as could 

 the same shoot through its own roots. In the case of 

 Linaria or Veronica, absorption through the root system 

 was as rapid as through a cut surface. There is an evident 

 disproportion between the parasite's water requirements and 

 the powers of the root to satisfy them, and in the increase 

 of the water and salt supply we see the chief advantage 

 derived from the parasitic attachment. In Euphrasia, 

 where independent existence is possible, the disproportion 

 between the supplying power of the root and the shoot's 



