168 PLANTS MODIFIED BY THEIR SURROUNDINGS 



single plants have evidently given rise to the members of each little 

 family group, and thus have populated the locality. 



So we find among plants communal conditions similar to those 

 among some animals. The many individuals of the community 

 live under similar conditions; they need the same substances from 

 the air, the water, the soil. They all need the light; they use the 

 same food. Therefore there must be competition among them, 

 especially between those near to each other. The plants which are 

 strongest and best fitted to get what they need from their surround- 

 ings live; the weaker ones are crowded out and die. 



But their lives are not all competition. The dead plants and 

 animals give nitrogenous material to the living ones, from which the 



latter make living matter ; 

 some bacteria provide cer- 

 tain of the green plants 

 with nitrogen ; many of the 

 green plants make food 

 for other plants lacking 

 chlorophyll, while some 

 algae and fungi actually 

 live together in such a 

 way as to be of mutual 

 benefit to each other. The 

 larger plants may shelter 

 the smaller ones, protect- 

 ing them from wind and 

 storm, while the trees hold the moisture in the ground, giving it 

 off slowly to other plants. Animals scatter and plant the seeds 

 far and wide, and man may even plant entire colonies in new 

 localities. 



How Plants invade New Areas. New areas are tenanted by 

 plants in a similar manner. After the burning over of a forest, we 

 find a new generation of plants springing up, often quite unlike the 

 former occupants of the soil. First come the fireweed and other 

 light-loving weeds, planted by means of their wind-blown seeds. 

 With these are found patches of berries, the seeds of which were 

 brought by birds or other animals. A little later, quick-growing 

 trees having seeds easily carried for some distance by the wind, 



A community of trilliums. Photograph 

 W. C. Barbour. 



by 



