GROWTH 



211 



this alternation of dark and light layers in every splinter, 

 even in a match. The microscope betrays the immediate 

 reason for this difference. This figure (fig. 62) shows a 

 small piece of pine wood, the transverse section of a 

 match. Across the middle of it runs the boundary 

 between two annual layers. 1 In the lower part lie 

 some summer and all the autumn cells, say of last 

 year ; in the upper the spring cells of the present 



FIG. 62. 



year. The sharp transition from the autumn to the 

 spring cells is easily perceived : the former have a 

 flat shape, thick walls, small and narrow cavities ; the 

 others are almost square, have thin walls and large 

 cavities. For a long time botanists could not account 

 for this change in the form of the cells laid down at 

 different seasons of the year, until they conceived the 

 idea that the fact must depend on the mutual pres- 

 sure, the mutual tension of the tissues. We made 



1 The transverse line dividing the section into two parts to the right 

 and to the left is the medullary ray (see previous chapter). 



