CHAPTER V 

 BRYOPHYTES 



THE FIRST LAND PLANTS 



42. Problem of the land habit. It was stated that Algae 

 live exposed to water as a medium, and that their structures 

 and habits are explained by this fact. To live on the 

 land means exposure to air as a medium, and the structures 

 and habits of land plants are explained by this fact. The 

 danger of exposure to air is the loss of water by the plant. 

 It must lose water to the drying air, and unless there is some 

 check, the loss will be greater than the supply, and death 

 will ensue. 



When an alga is removed from water and exposed to air, 

 it dries out quickly and perishes, for there is no check to the 

 very rapid loss of water from the protoplasts. Therefore, 

 if water plants are to become land plants, they must acquire 

 the air-habit by developing protective structures. It is 

 believed that this is just what certain Algse did, and that in 

 so doing they became so different in structure that they 

 ceased to be Algae. In any event, the acquiring of the 

 land-habit was about the most important happening in the 

 history of plants, for it made possible all the subsequent 

 progress of the plant kingdom. 



One may picture how a gradually increasing exposure to 

 air might have occurred, beginning with occasional exposures 

 on muddy flats, and by gradual shoreward migration, ending 

 in continual exposure. Even when exposure to air became 

 continual, it must have been for a long time in conditions of 



