80 BOTANY. [chap. n. 



readily accommodating themselves to very marked 

 changes, others being rigid, either entirely disappear or 

 retire into the background when placed under similar 

 conditions. A second reason in support of the idea that 

 parasitism is an acquired function that has been gra- 

 dually evolved, and is still going on at the present day, 

 is the transitional forms from chlorophyll-producing 

 plants to others entirely destitute of chlorophyll. The 

 well-known mistletoe (Viscum album) is an example of 

 such a transitional species, being a parasite to the extent 

 of obtaining from the host upon which it grows all the 

 food and water taken from the soil by the roots of the 

 host plant; but the leaves of the mistletoe contain 

 chlorophyll, hehce it still retains and exercises the pro- 

 perty of taking in carbonic dioxide from the air, and of 

 producing starch in the normal manner. The stage of 

 parasitism reached by this plant is that of taxing a host 

 plant to supply the required amount of water with food 

 substances in solution, instead of taking them directly 

 from the soil for itself. In the ^g wort order (^Scrophu- 

 lariacece), as understood in the broader sense, we have 

 even in British species an interesting sequence in the 

 evolution of parasitism; the species of eyebright 

 [Einphrasia) , yellow rattle {Rhinanthus) , lousewort 

 (Pediculcms) , and Bartsia, all common plants of our 

 meadows and moorlands, have leaves furnished with the 

 normal amount of chlorophyll, and, so far as general 

 appearance goes, appear to obtain the whole of their food 

 in the manner normal to ordinary plants ; but in reality 

 all are to a certain extent parasites, or, as one might say, 

 have broken away from the normal plant-mode of ob- 

 taining food, and resorted to a device for obtaining a 



