56 PLANT GEOGRAPHY 



leaves, but have generally to produce an exceptional 

 amount of seed, owing to the risk it runs of not reaching 

 a suitable spot for germination. They vary as to the 

 number of different hosts upon which they are capable 

 of growing; but obviously their distribution depends 

 upon that of the hosts. This is, to a less extent, the 

 case with such partial or " facultative " parasites as 

 the Cow-wheats (Melampyrum). 



In epiphytes, such as the Algae, Lichens, and Mosses 

 that grow on the trees of temperate latitudes, and the 

 Orchids, Aroids, Bromeliads, Peppers, etc., on those of 

 the tropics, as there is no physiological attachment to 

 the host plant, there is less restriction as to what species 

 it shall be. This mode of life produces, however, 

 marked structural characters, such as the green aerial 

 roots, sometimes covered with a velamen or water- 

 absorbing tissue, the water-holding pitchers of Dischidia, 

 and the humus-collecting " pocket-leaves " of the Elks- 

 horn Fern (Platy cerium alcicorne Desv.). A similar 

 relation to the plants upon which they occur is that of 

 the marvellous rope-like " lianes " of the tropical forests. 

 Belonging to a number of different orders, such as 

 AmpelidacecB, ApocynacetB, Asclepiadacecs, BignoniacecB , 

 DioscoreacecB , etc., they agree generally in the long inter- 

 nodes of shade-plants, in their long-stalked and often 

 cordate leaves. They differ from true epiphytes in 

 being rooted in the ground. 



The brown saprophytic flowering plants, whether 

 dicotyledonous, like Monotropa, or monocotyledonous, 

 like Neottia, with their reduced roots, leaves, and vascular 

 systems, are but slightly dependent on the trees amongst 

 whose dead leaves they grow, and neither benefit nor 

 harm them. It is, however, interesting to note the 

 unexplained fact that Monotropa Hypopitys L. is 

 pubescent when growing under conifers, smooth when 

 under broad-leaved trees. 



COMMENSALISM. Plants of normal nutrition, with 

 normal root-systems and green leaves, merely growing 

 side by side, and having, therefore, apparently similar 

 requirements as to climate and soil, are termed com- 

 mensals (table-companions); and obviously the keenest 

 physiological competition will be between such plants 

 when they are " like," i.e. of the same or closely-related 

 species. These " social " plants, as they are termed, 



