Adaptation to Environment. 305 



support the respiration necessary to the life of the proto- 

 plasts, and the plants would drown. 



But beside these dying plants others, native to the water, 

 would be thriving, well able to obtain their necessary oxy- 

 gen from the relatively small amount in the water, because 

 their bodies (in the case of Ceratophyllum and Myriophyl- 

 lum, for instance) are divided into slender segments, which, 

 Uke the gills of a fish, expose a large surface for absorp- 

 tion ; and because the very slight amount or total absence 

 of waterproofing on their surfaces permits a ready inter- 

 change of gases. 



If, however, we remove the water plant and place it on 

 the land, it also soon dies ; but now not for lack of oxygen, 

 for it can obtain more of this than before, but for lack of 

 sufficient water, since the plan of its construction which 

 adapted it to the water now allows it quickly to dry up. 

 It is clear from these illustrations that if a plant is to suc- 

 ceed in life, or if it is to live at all, it must be adapted by 

 its form and construction to the habitat in which it finds 

 itself. 



186. Kinds of Habitats. — We find some plants living in 

 the water, others on the land, that is, with roots in the soil 

 and shoots in the air ; and still others entirely in the air 

 without connection with the soil, such as some tropical 

 orchids. If we examine any one of these classes of habi- 

 tats, we find that it offers wide ranges of variation. The 

 water may be deep or shallow, still or running, hot, as in 

 hot springs, or cold, fresh or salt, turbid or clear. The 

 soil may vary greatly as to its chemical composition or 

 physical condition; it may be clayey, loamy, sandy, 

 gravelly, rocky, etc. ; it may contain little or much of com- 

 mon salt ; it may be warm or cold, depending in part on 

 the foregoing conditions. The air may be relatively quiet 



