THEIR PLACE IN NATURE 



251 



473. — A seaweed 

 with broad expanded 

 thallus. 



356. Classification. — Beginning with the lowest forms, 

 seedless plants are grouped into three great orders, or 

 classes. 



357. I. Thallophytes, or thallus plants. 

 This group takes its name from the 

 thallus structure that characterizes its 

 vegetation. What a thallus is will be 

 better understood after a specimen has 

 been examined. It may be stated, how- 

 ever, that the term is applied in general 

 to the simplest kinds of vegetable struc- 

 ture, in which there is no differentiation 

 of tissues, and no true distinction of 

 root, stem, and leaves. While it is not 

 peculiar to the thallophytes, it has 

 attained its most typical development among them, and 

 the name is therefore retained as distinctive of that group. 

 It embraces two great divisions, the Algae and Fungi. 

 The first includes seaweeds and the common fresh-water- 

 brook silks, pond scums, etc., besides numerous micro- 

 scopic forms whose presence escapes the eye altogether, 

 or is made known only by the discolorations and other 



changes they effect in the water. 



To the fungi belong the mushrooms 

 and puff balls, the molds, rusts, 

 mildews, etc., and the vast tribe of 

 microscopic organisms called bac- 

 teria, that are so active in the pro- 

 duction of fermentation, putrefaction, 

 and disease. 



^xX--^ 



474. — Anthoceros, s 

 liverwort with flat, spread 

 ing thallus. 



358. II. Bryophytes, or moss plants. 

 This group likewise contains two 

 divisions, mosses and liverworts. 

 Familiar examples of the latter are 

 the marchantia, or umbrella liverwort (Figs. 500, 502), 

 commonly found on the ground in cool bogs, and the flat, 

 spreading plants, bearing somewhat the aspect of lichens. 



