BRYOPHYTES 67 



shade and moisture, for life " in the open " means extreme 

 danger from loss of water. 



43. The Liverworts. This group of plants is not con- 

 spicuous, and therefore it is not noticed by most people, 

 but it is believed to be the group that acquired the land- 

 habit and that has given rise to all the higher plants. The 

 Liverworts, therefore, are of very great historical importance, 

 and should be known, by sight at least, to every student of 

 plants. They may be called the amphibians of the plant 

 kingdom, for they connect the water forms with the land 

 forms. 



Although Liverworts solved the problem of living on land, 

 they give one the impression of being on the defensive against 

 the air rather than of using the air and sunshine freely. For 

 the most part, they occur in moist places, or at least in shaded 

 places. It is one thing to live in protected places, and a very 

 different thing to live and work in the open. To occupy 

 the general land surface means that many plants must live 

 in unprotected places, and it remained for the higher plants 

 to solve this problem, by substituting protective structures 

 and habits for protected places. 



There are two general facts in this connection that should 

 be kept in mind. One is that a plant may work in a pro- 

 tected place, but the opportunities for work are not so great. 

 The progress of plants, therefore, implies that they must 

 acquire the ability to use the largest opportunities. An- 

 other fact is that work and endurance are not the same 

 thing. Plants may endure conditions of exposure, but not 

 be able to work except so far as it is necessary to maintain 

 life. An ordinary deciduous tree endures the winter, but it 

 works in the summer. In considering the progress of plants, 

 therefore, it is not a question of the conditions which they 

 can endure, but of the conditions in which they can work. 



44. The body of Liverworts. If Liverworts are Algae 

 that have acquired the land-habit, it is important to know 



