166 PLANTS MODIFIED BY THEIR SURROUNDINGS 



inches. Conditions of moisture in the country cause great differ- 

 ences in the plant covering. 



In the eastern part of the United States the rainfall is sufficient 

 to give foothold to great forests, which aid in keeping the water in 

 the soil. In the Middle West the rainfall is less, the prairies are 

 covered with grasses and other plants which have become adapted 

 to withstand dryness. In the desert region of the Southwest we 

 find true xerophji;es, cacti, switch plants, yuccas, and others, all 



A rock society. Photograph by W. C. Barbour. 



plants which are adapted to withstand almost total absence of 

 moisture for long periods. In the Temperate Zone the water supply 

 is the primary factor which determines the form of plant growth. 

 Plant Formations and Societies.^ — All of the factors alluded to 

 act upon the plants we find living together in a forest, a sunny 



I Plant Societies. Field Work. — Any boy or girl who has access to a vacant lot 

 or city park can easily see that plants group themselves into societies. Certain 

 plants live together because they are adapted to meet certain conditions. Societies 

 of plants exist along the dusty edge of the roadside, under the trees of the forest, 

 along the edge of the brook, in a swamp or a pond. It should be the aim of the 

 field trips to learn the names of plants which thus associate themselves and the condi- 

 tions under which they live, and especially their adaptations to the given conditions. 

 Suggestions for such excursions are found in Andrews, Botany All the Year Round; 

 Lloyd and Bigelow, The Teaching of Biology ; and Ganong, The Teaching Botanist. 

 A convenient form for an excursion is found in Hunter and Valentine, Manual, page 

 202. This trip may be taken in the early fall. 



