LESSON XXVIII 



MOSSES. 



IN the four previous lessons we have traced the advance 

 in organization of animals from the simple diploblastic 

 Hydra to the complicated triploblastic forms which con- 

 stitute the five higher phyla of the animal kingdom. We 

 have now to follow in the same way the advance in structure 

 of plants. The last member of the vegetable kingdom with 

 which we were concerned was Nitella (p. 206), a solid 

 aggregate, exhibiting a certain differentiation of form and 

 structure, but yet composed of what were clearly recognizable 

 as cells, there being, as in Hydra, none of that formation of 

 well-marked tissues which is so noticeable a feature in 

 Polygordius as in other animals above the Ccelenterata. 



Taking Nitella as a starting point, we shall see that among 

 plants, as among animals, there is an increasing differentiation 

 in structure and in function as we ascend the series. The 

 first steps in the process are well illustrated by a considera- 

 tion of that very abundant and beautiful group of plants, the 

 Mosses. In spite of the variations in detail met with in 

 different genera of the group, the essential features of their 

 organization are so constant that the following description 

 will be found to apply to any of the common forms. 



