HISTORY OF THE EARTH 293 



(1) AI/LE, FUNGI, BACTERIA, ETC. (Thallophytes). 



The simplest of all plants. Among them are seaweeds, 

 diatoms, molds, yeast, mushrooms, etc. They have 

 no distinct roots or leaves, and they are reproduced, 

 not by seeds, but by minute germs or spores. Some 

 merely divide, each part then becoming a distinct plant. 

 The majority of thallophytes live in the water or in 

 very moist places, and some are mere single cells of 

 jellylike substance, too small to be seen with the naked 

 eye. The fungi, bacteria, and many others are not 

 green in color. 



(2) Moss GROUP (Bryophytes). 



More advanced plants than the last, in that some of 

 them have definite leaves and stems. But true seeds 

 and flowers are still lacking. They include the mosses 

 and liverworts, all small, delicate plants. 



(3) FERN GROUP (Pteridophytes). 



In the ferns and their allies for the first time we find 

 a system of tubes and pores which allow the circulation 

 of sap and air within the plant. Some ferns are treelike 

 and have woody tissues, but most of them are small 

 herbs. Geologically the chief interest now centers in 

 a group which we may call the seed ferns (Pterido- 

 spermae), 1 all of them now extinct. These had all 

 the appearance of true ferns, and were formerly 

 regarded as such; but it is now known that they 

 possessed fruitlike organs with true seeds, which indi- 

 cate a close relationship with the next group. They 

 seem to be intermediate in many respects between 

 ferns and cycads. 



1 The older classifications of plants have recently been modified in con- 

 sequence of the studies of Seward, Scott, and others on fossil plants. The 

 scheme here used is adapted partly from Scott's writings. 



