90 THE LESSON OF EVOLUTION 



going on. In the first place special reproductive 

 cells became enclosed in special reproductive organs. 

 Then followed the differentiation of the other organs : 

 stem, leaf, and root in plant ; alimentary, muscular, 

 circulatory, respiratory, and nervous systems in 

 animals. This was accompanied by unequal division 

 of the cells ; the two daughter cells often differing 

 from each other in size or in shape, or in potential 

 energies. 



As a simple example of the growth and develop- 

 ment of an organism, I will take Nitella, one of the 

 CharaceaB or Stone-Worts submerged water plants 

 not uncommon in limestone districts. 18 



The germinating oopore is oval, and its first step 

 is to divide into two portions of very unequal size, 

 separated by a cell-wall at right angles to the longer 

 axis of the spore. The larger of these cells grows no 

 more, but merely supplies food-material for the 

 smaller one. This latter divides by a cell-wall per- 

 pendicular to the first. One of these daughter cells 

 grows into the primary root, the other into a short 

 primary stem, or proembryo. Upon this primary 

 stem a lateral bud appears, which grows into the 

 plant. The stem of the plant increases in length 

 by the apical cell continually cutting off segments 

 transverse to the axis. Each of these segments 

 immediately divides into two halves by a cell-wall, 

 also transverse to the axis of the stem, and the lower 

 of these halves elongates into an internode filled with 

 cell-sap ; the protoplasm forming a lining to the cell 



further details and figures, see Jeffery Parker's 

 " Elementary Biology," Macmillan and Co. 



