DEVELOPMENT OF PLANTS 341 



places. This peculiar distribution of the species of Equisetum 

 has not been explained. It is evident that these plants, like the 

 rushes and sedges of our marshes and shallow ponds, are often 

 exposed to intense heat and light, which would cause an excessive 

 transpiration. Possibly these plants are not able to absorb 

 water rapidly owing to the limited amount of conducting tissues 

 and to the exclusion of the atmosphere by the water which sur- 

 rounds the roots (see pages 45, 54), consequently they are at 

 the same disadvantage and require the same protective devices 

 as plants living in arid localities. 



Order 4. Lycopodiales. The Club Mosses 

 115. General Characters. The members of this group are 

 popularly known as the club mosses owing to their small moss- 

 like leaves and the arrangement of the spore-bearing leaves into 



FIG. 236. A common club moss, Lycopodium annotinum, with creeping 

 stem and erect branches covered with small moss-like leaves: s, strobilus, 

 r, roots. H. O. Hanson. 



