194 TBE NATURE AND WORK OF PLANTS 



ments have been followed in the development of 

 human societies. The hunter and the pioneer first 

 occupy the soil or land, to be followed by the miner 

 and the farmer, and then by the manufacturing town 

 or the city. 



234. Water and plants. — The plant is ex- 

 tremely sensitive about its supply of water. Not 

 only must it have a certain amount of rainfall each 

 year, but the rain must be given throughout the 

 whole year, instead of all at one time. The amount 

 capable of being retained by the soil and the salts 

 it contains are also of importance. The pond socie- 

 ties float in or on top of the water, or are submerged 

 beneath it. 



235. Temperature and plants. — Each species 

 requires a certain number of warm days for its 

 development and can endure only certain low tem- 

 peratures. Thus our summers in the Middle states 

 are warm enough for the castor-oil plant out of doors, 

 but the low temperatures of winter kill it. The 

 warmth-retaining value of the soil is to be taken 

 into account in this connection. The surface of the 

 country is also a feature. In broken hilly regions 

 the cold air settles down in the valleys, making 



