ECOLOGICAL GEOUPS 479 



structure and mode of activity of the plant or organ under 

 discussion. From the point of view of the ecologist the prin- 

 cipal things to be considered are the relations (often very 

 intricate) between the plant and other plants or animals, how 

 the plant meets the conditions of soil and climate under which 

 it exists, and why plants are distributed m the various regions 

 of the earth's surface as they are. 



440. Systematic groups not ecologicaL The way in which 

 plants are classified according to their relationships has already 

 been described (Chapter X). The systematic grouping into 

 classes, orders, families, and so on, has no necessary relation 

 to the life habits of the plant. The Heath order, for exam- 

 ple, includes ordinary plants with the capacity for photosyn- 

 thesis (Fig. 299), and also saprophytes (Figs. 307 and 308). 

 Some of its members are sun-loving plants and some are shade- 

 enduring, some can live in very dry soil and others occur only 

 in swamps. One familiar genus of the Morning-glory family 

 (^Cuscuta, Fig. 351) is parasitic, while most of the genera of 

 this family get their living in the ordinary way, from air and 

 soil. Several genera of the Figwort family (such as the pamted 

 cup) are root parasites, while most are not. 



441. Ecological groups. In classifymg plants according to 

 their ecological relations they are generally grouped with 

 regard to their water requirements, as follows: 



(1) Water plants, those which usually live only in the water ^ 

 or in marsh soil saturated with water ^; they may be unattached, 

 like many algae (Chapters XII, XIII) and duckweed (Fig. 

 857) ; or rooted, like arrowhead (Fig. 359), cat-tails, pickerel 

 weed, and pond lilies (Figs. 55 and 358). 



(2) Land plants, those wliich hve in ordmary soil or on 

 rocks, the bark of trees, and so on. 



The two main ecological divisions of land plants are as 

 follows ^ : 



(rt) Xeropht/tes, plants which can tolerate extremely dry 

 conditions, as many lichens (Figs. 190-193), cacti (Fig. 6S), 



1 Hydrophytes. ^ Helophytes. ^ For halophytes see Sect. 452. 



