PARTIAL PARASITES 219 



the 4 species of which are large, fleshy root parasites, {k) Cus- 

 cuta, with about 100 species, all twining parasites, is an 

 isolated parasitic genus belonging to the Convolvulaceae. 

 (/) The 2 species of Krameria are parasites on the roots of 

 shrubs of the Arizona deserts. The genus is of uncertain 

 systematic position, but may be referred to the Leguminosce. 



Partial Parasites. — All parasites do not show the same 

 degree of dependence on their hosts. Complete parasites 

 possess no chlorophyll and must draw a supply of elaborated 

 organic food from the host. They show a great range of 

 reduction of their vegetative organs. As partial parasites we 

 distinguish those which do possess chlorophyll, and which 

 probably absorb only water and salts from the tissues of 

 the plant on which they live. Such partial parasites as the 

 mistletoe can none the less develop only in connection 'with 

 a suitable host, while others, like the yellow rattle, can come 

 to maturity as independent plants. We have, therefore, all 

 grades from facultative parasites, which may grow inde- 

 pendently, and which differ little, if at all, from normal 

 plants, to such advanced parasites as Rafflesia, in which 

 complete dependence on the host is accompanied by loss 

 of chlorophyll, and extreme reduction of the vegetative 

 organs. The Rhinanthoidese are particularly instructive. 

 The tribe includes independent genera, facultative parasites 

 such as the yellow rattle, complete parasites such as the 

 tooth-w^orts, and genera showing intermediate stages. We 

 may begin a more detailed survey with this group. 



Rhinanthoidese. — Among the little specialised partial 

 parasites are such common British plants as the cow-wheats 

 (Melampyrum), the yellow rattles (Rhinanthus), the eye- 

 brights (Euphrasia and Bartsia), and the red rattle (Pedicu- 

 laris). These plants all possess green foliage though it 

 may be rather scant, the plant giving an impression of 

 scragginess ; they also have a normal though rather poorly 

 developed root system with root hairs. They do not 

 appear very different from other common plants among 

 which they grow. The seeds germinate easily and, if sown 

 in soil kept free of other plants, they produce free-living 



