I.] GENERAL CHARACTERS AND STRUCTURE, n 



direction of the radius of the stem ; and the chisel- 

 edge must be supposed to run in the direction parallel 

 to that of a medullary ray, i.e. radially. From the 

 first, each cambial cell contains protoplasm and a 

 nucleus, and is capable of being nourished and of 

 growing and dividing. It is only at or near the tips 

 of the branches, &c., that these cambium-cells are 

 growing much in length, however ; and in the parts 

 we are considering they may be for the most part 

 regarded as growing only in the radial direction ; 

 more rarely, and to a slight extent, in the tangential 

 direction also, as the circumference of the cylinder 

 enlarges. After a cambial cell has extended the 

 superficial area of its walls by growth in the radial 

 direction to a certain amount, a septum or division 

 wall arises in the longitudinal tangential plane, and 

 two cells are thus formed in place of one : this process 

 of division may then be repeated in each cell, and so 

 the process goes on. This is not the place to lay stress 

 on certain facts which show that a single layer of cells 

 probably initiates the division : it suffices to point out 

 that by the above process of division of the cambial 

 cells there are formed radial rows of prismatic cells, 

 as indicated in Fig. 5, where the arrow points along a 

 radius towards the centre of the stem. It is true such 

 radial rows of cells are also developed in smaller 



