302 



HOW CROPS GROW. 



structure of which, while the cell is young, is shown by 

 a section through them lengthwise. A exhibits such a 

 section through the thickened walls of two contiguous 

 and adhering cells, x, in both A and B, shows a cavity 

 between the two primary cell-walls ; y is the narrow 

 part of the channel, that 

 remains while the mem- 

 brane thickens around it. 

 This is seen at y, as a pit 

 in each cell-wall, or, as 

 Schacht believed, a pore 

 or opening from cell to 

 cell. In A it appears 

 closed because the section 

 passes a little to one side 

 d! the pore. (Schacht.) 



In the next figure (54), 

 representing a transverse 

 section of the spring wood 

 of the same tree magnified 

 300 diameters, the struct- 

 ure and the gradual form- 

 ation of these pore-disks 

 is made evident. The sec- 

 tion, likewise, gives an in- 

 structive illustration of 

 the general character of the 

 simplest kind of wood. R -. 

 are the young cells of the " 

 rind ; C is the cambium, 

 where cell-multiplication 

 goes on; W is the wood, whose cells are more developed 

 the older they are, i. e., the more distant from the cam- 

 bium, as is seen from their figure and the thickness of 

 their walls. At a is shown the disk in its earliest stage ; 

 I and c exhibit it in a more advanced growth. At d the 



