270 Science of Plant Life 



on rock surfaces, and occasionally on the soil (Fig. 160). Like 

 other fungi they produce fruiting bodies, — small cup-shaped 

 or disklike elevations, — in which asexual spores are pro- 

 duced in great numbers. 



Summary of the simple plants. The simplest forms of 

 plant life include three great groups that are of the highest 

 importance to man : 



(i) The algae constitute the primary food of fishes, and 

 they will become increasingly important as the cultivation 

 of ponds, lakes, and streams (aquaculture) for the production 

 of fish becomes more necessary to augment our food supplies. 



(2) The bacteria have a most important and intimate bear- 

 ing upon the lives of all of us. They have made necessary 

 our various food-preserving industries. Their existence every- 

 where is the chief factor that must be considered in personal 

 hygiene and in the making of our sanitary laws. Some kinds 

 of bacteria aid other forms of life by destroying the bodies of 

 dead organisms and improving the fertility of soils; other 

 bacteria are responsible for many diseases of both plants and 

 animals. 



(3) The fungi also are destructive agents, causing injury 

 to our crop plants, the loss of much valuable food, and the 

 destruction of timber. Forms like the yeasts are valuable 

 aids in certain food industries. A very few of the fungi are 

 themselves sources of food. The fungi are notable for the 

 enormous number of spores which they produce. 



Algai are autophytes, while the bacteria and fungi are 

 saprophytes or parasites. The algae are of world-wide dis- 

 tribution, but they are chiefly confined to aquatic and marine 

 habitats. The bacteria and fungi are also world-wide in their 

 distribution, but they are most numerous on land. 



