126 BRITISH PLANTS 



ence of the parasite upon the host may be partial or 

 complete : 



(i.) Partial or Hemi-Parasites. These possess chloro- 

 phyll, and so can make carbohydrate for themselves. 

 They are generally found attached to the roots of other 

 plants, which they tap for water ; but, like other para- 

 sites, they take all they can get, and though water is their 

 chief need, food may be absorbed as well. The British 

 partial parasites, with the exception of Thesium humi- 

 fusum (bastard-toadflax), belong to the Natural Order 

 Scrophulariaceae, and include the following plants, which 

 live attached to the roots of grasses by means of haus- 

 toria : Euphrasia (eyebright), RJiinanthus (yellow rattle), 

 Pedicularis (lousewort), Melampyrum (cow-wheat), and 

 Bartsia. All these plants live in wet grass-communities. 

 Grass-roots form a turf so thick that no other shallow- 

 rooted plant has a chance in competing with them for 

 water. Daisies, dandelions, and other plants growing in 

 meadows have long roots which penetrate the turf, and 

 so they are not really in competition with the grasses. 

 These hemi-parasites, however, are shallow-rooted. Being 

 forced, then, to compete with the grass-roots, they solve 

 the difficulty by constraining their rivals to their service. 

 They fix their own roots upon theirs, and tap them for 

 water. But that they take something more than water 

 is proved by the fact that, in a field where yellow rattle 

 is abundant, the grass is poor and sickly, and much of it 

 dies before the season when it should be mown for hay. 



(ii.) Total Parasites. These are devoid of chlorophyll, 

 and depend entirely upon their hosts for food. In the 

 British flora the following are total parasites among the 

 flowering plants : Lathrcea (toothwort), Orobanche (broom- 

 rape), Cuscuta (dodder). The first two belong to the 

 Orobanchaceae, a Natural Order which differs very little 

 from the Scrophulariaceae, in which most of the partial 

 parasites are included. 



(1) Lathrsea squamaria (Fig. 41). The toothwort is 

 parasitic upon the roots of trees, chiefly poplars, hazels, 

 and beeches. Its body consists of a thick, much-branched 

 rhizome, closely set with curious fleshy, tooth-like scales, 

 of a grey or dusky- purplish colour, overlapping each other 

 and arranged in four ranks. Each scale is hollow, en- 

 closing an irregular chamber, which is open to the exterior 



