PARASITISM 237 



Research on water relations has just begun. Here is a wide, 

 and it should be added a difficult, field to be explored. 



An enticing problem, more of a speculative nature, is 

 the origin of the parasitic habit. The distribution of 

 parasitic groups in the system of flowering plants shows that 

 the habit has certainly arisen more than once in evolu- 

 tionary history, but that it has not arisen very frequently. 

 Cassytha, Cuscuta, and Krameria are parasitic genera in 

 otherwise autotrophic families, which are not related to any 

 of the other groups of parasites. In the Scrophulariaceas 

 the habit appears in various genera of a single tribe ; the 

 closely related Orobanchaceae are completely parasitic. 

 The small family of Lennoceae is not related to any other 

 parasitic family. The Santalaceae and Myzodendraceae are 

 related, as are the Loranthaceae and Balanophoraceae, and 

 the Hydnoraceae and Rafflesiaceae. These families stand 

 near each other in the natural system. It may be noted 

 that there are no parasitic monocotyledons nor gymno- 

 sperms. It looks as if in certain plant groups there has been, 

 and probably still is, some tendency of an unexplained 

 nature which has made parasitic development possible. 

 This tendency is present in the Rhinanthoideae, and in the 

 very closely related Orobanchaceae. It shows itself in the 

 group of families Loranthaceae, Balanophoracese, Hydno- 

 raceae, and Rafflesiaceae, and in the Santalaceae and Myzo- 

 dendraceae. It crops out quite unexpectedly in the isolated 

 genera Krameria, Cassytha, and Cuscuta. It may be that 

 the nature of the tendency in the various cases has been 

 quite diflferent. It is certain that in every case it has been 

 the acquisition of some positive quality or qualities which 

 has made possible the connection with the host. Para- 

 sitism has almost certainly not been a consequence of 

 diminished capacity for independent existence, nor of 

 reduction of vegetative structure ; these have followed. 

 Parasitism, in other words, is primarily a positive capacity, 

 and only secondarily does it mean degeneration. 



A factor in the evolution of parasitism has been oppor- 

 tunity. Parasitism in Cuscuta has been favoured by the 



