ORGANIC EVOLUTION 121 



stratum where they are in communication with the air 

 through the pores, and with sunlight, which pene- 

 trates the transparent epidermis. These differ among 

 themselves in form according to their situation. 



2. Common parenchyma, of bulky, colorless cells com- 

 posing the thicker interior stratum, which gives form to the 

 thallus. A few common parenchyma cells rise above the gen- 

 eral level, passing between groups of assimilatory cells to 

 mark out the areas already seen from the surface. 



These are the primary tissues of all the higher plants. 



Such integration of cells into a unit organism of mutually 

 dependent parts, we have not found before. In the algae we 

 have studied, the cells are more loosely associated together, 

 less differentiated, and physiologically more independent. 

 When every cell is in contact with the water that contains 

 its food, there is no need of special feeding organs. 



But with the assumption of terrestrial life, the sort of 

 division of labor that we have just been considering has 

 taken place among the cells of the liverwort. Different 

 plant functions are assumed by different groups of 

 cells; that of getting carbon from the air by the cells of 

 the assimilatory parenchyma ; that of getting the mineral 

 matters from the soil, mainly by the rhizoid cells; that of 

 protecting the protoplasm from the new peril of evaporation, 

 by the cells of the epidermis, etc. ; each group doubtless 

 improving in capacity for its special work, as it is relieved 

 of the work now performed by other cell groups. 



Cell division is localized in the liverworts, and in all the 

 higher plants." It occurs in Conocephalus only at growing 

 points located in the notched tips of the stem and branches. 

 There new cells are formed during the growing season. 

 They are at first minute and rich in protoplasm. They 

 rapidly increase in size and differentiate into the kinds of 

 tissue just described, and take their places, to remain of one 

 form until the death of the stem. 



