THE PAST HISTOBY OF PLANTS. 227 



bacteria, which are one-celled organisms, living 

 in stagnant or putrid fluids, and also in the 

 bodies and blood of diseased animals. They 

 answer among fungi to the one-celled alga. 

 Many of them cause infectious diseases ; such 

 are the bacilli of diphtheria, typhus, cholera, 

 consumption, small-pox, and influenza. Sur- 

 rounded by a suitable nutritious fluid, these tiny 

 parasitic plants increase with extraordinary and 

 fatal rapidity. Though they are really one- 

 celled, and reproduce by cell-division, they often 

 hang together in rude lumps or clusters which 

 simulate to some extent the many-celled bodies. 

 In this book, however, where we have concen- 

 trated our attention mainly on the true or green 

 plants, I have not thought it well to dwell at 

 any length on the habits or structure of these 

 animal-like organisms. 



Another well-known group of small fungus- 

 like plants is that which contains the yeast- 

 fungus, a one-celled plant, which reproduces by 

 budding. 



The higher fungi are many-celled, and often 

 possess well-marked organs for different pur- 

 poses. They answer rather to the seaweeds 

 and higher alga. Familiar examples are the 

 common moulds, which form on jam, dead 

 fruit, and other decaying material. Some of 

 them, like the smut of wheat and oats, are 

 parasitic on growing plants, and most dangerous 

 enemies to green vegetation. The highest fungi 

 are the groups which include the mushroom, 

 the puff-ball, and all those other large and 

 curiously-shaped forms commonly lumped to- 



