[Chap. I PLANT SCIENCE 



Fig. 3. Brown seaweeds (Algae) at low tide. New York Harbor. Photo from New 



York Botanical Garden. 



soil at times become green with "pond scums" — microscopic plants that 

 may be distinguished as al^ae ( Fig. 3 ) . Those who have visited the sea- 

 shore are familiar with "seaweeds," which are other and larger kinds of 

 algae. Tree trunks and rocks that have long been exposed to the weather 

 are often partially covered by gray-green or highly colored patches of 

 lichens ( Fig. 4 ) . Everyone who even occasionally strolls on forest trails 

 distinguishes some of the mosses (Fig. 5) and ferns (Fig. 6). Much of 

 the decay of plant and animal products, and many of the diseases of 

 both plants and animals are due to the presence of bacteria ( Fig. 7). All 

 these organisms are plants. 



On first thought it may seem rather easy to answer the question: What 

 is a plant? But one may be thinking only of such plants as trees, shrubs, 

 and herbs. When we include in our survey mosses, algae, fungi, and 

 bacteria, the observable differences among plants are often far more strik- 

 ing than those between certain plants and certain animals. This fact 

 makes it surprisingly difficult to list any group of qualities that are char- 

 acteristic of plants alone. 



Similarity of plants and animals. The larger plants differ from the 



