PLANT ECOLOGY 315 



less amount than others, and are " shade plants " as dis- 

 tinct from " light plants." In forests and thickets many 

 of these shade plants are to be seen which would find an 

 exposed situation hard to endure. In almost every associa- 

 tion, therefore, plants are arranged in strata, dependent 

 upon the amount of light they receive, and the number of 

 these strata and the phmts characterizing each stratum are 

 important factors to note. 



Wind. — This is an important factor in regions where 

 there are strong prevailing winds. Wind has a drying 

 effect and increases the transpiration of plants, tending to 

 impoverish them in water. In such conditions only those 

 plants can live which are well adapted to regulate trans- 

 piration. 



The above five factors are among the most important, 

 but no single factor determines an association. As each 

 factor has a large possible range, the combinations of fac- 

 tors may be very numerous, and it is these combinations 

 which determine associations. For convenience, however, 

 associations are usually grouped on the basis of the water 

 factor, at least three great groups being recognized. 



179. Hydrophyte associations. — These are associations of 

 water plants, the water factor being so conspicuous that the 

 plants are either submerged or standing in water. A plant 

 completely exposed to water, submerged, or floating, may 

 be taken to illustrate the usual adajDtations. The epi- 

 dermal walls are thin, so that water may be absorbed 

 through the whole surface ; hence the root system is very 

 commonly reduced or even wanting ; and hence the water- 

 conducting tissues (xylem) are feebly developed. The tis- 

 sues for mechanical support (stereome) are feebly devel- 

 oped, the plant being sustained by the buoyant power of 

 water. Such a plant, although maintaining its form in 

 water, collapses upon removal. Very common also is the 

 development of conspicuous air passages for internal aera- 

 tion and for increasing buoyancy ; and sometimes a special 



