PARASITES AND SAPROPHYTES 179 



saprophytic Bird's-nest Orchid). The haustoria of parasites 

 arise from the stem (Dodder, Cuscuta, Fig. 95) or root (Cow- 

 wheat, Melampyrum) as outgrowths (h.), which flatten out in a 

 sucker-like manner in contact with the surface of the host. The 

 centre of the sucker grows out as a peg-like process, which pene- 

 trates the cortex and ultimately reaches the vascular tissue of 

 the host, where it often widens out in a fan-shaped manner 

 (Fig. 95, h.). At first this process consists of undifferentiated 

 cells, but later those adjacent to the xylem become tracheids, 

 whilst those in contact with the phloem develop as phloem-like 

 elements. 



By the connection thus established between the conducting 

 elements of host and parasite, the assimilatory products and 

 absorptive system of the former become available for the parasite, 

 which regulates any excess of water thus obtained by means of 

 the hydathodes already described (cf. p. 144). The parasite, 

 being relieved of those functions which necessitate elaboration 

 of the vegetative structure, is enabled to utilise almost its entire 

 energies for the purpose of reproduction. Simplification of 

 structure, in every other direction but that which tends towards 

 an increase in the output of seeds, is a marked characteristic of 

 these organisms. Even the ovules (cf. p. 365) and embryos are 

 simpler in construction than those of most green plants, the 

 material economised in this way presumably making possible a 

 quantitative increase. 



