i84 



NATURE 



{Dec. 22, 1887 



supply abundance of nutriment to the growing cambium- 

 cells, it is not surprising that these cells cannot extend 

 themselves so far in the radial direction {i.e. in a line 

 towards the centre of the compressed stem), and that 

 their walls are thickened by richer deposits of woody 

 material supplied quickly to them. 



As the winter approaches, the cambium ceases to be 

 active, and it then remains dormant for several months. 

 When its cells are awakened to renewed growth and 

 division in the following spring, they at once begin to 

 form the tracheides with thin walls and large lumina, and 

 it is the sharp contrast thus displayed between the newly- 

 formed tracheides with thin walls and large lumina, and 

 the compressed denser ones on which they suddenly abut, 

 that produces the impression of the " annual ring." 



It is now time to attempt to give some clearer ideas 

 of what this " cambium " is, and how its cells become 

 developed into tracheides. But first it is necessary to 



point out that each tracheide is a long, more or less 

 tubular and prismatic body, with bluntly tapering ends, 

 and the walls of which have certain peculiar markings and 

 depressions on them, as seen in Fig. 4. We cannot here go 

 into the important signification andfunctions of these mark- 

 ings and depressions however, since their study would need 

 an article to themselves. It must suffice for the present 

 to state that the markings have reference to the minute 

 structure of the cell- walls, and the depressions are very 

 beautiful and complicated pieces of apparatus to facilitate 

 and direct the passage of water from the cavity of one 

 tracheide to that of another. Now, the cambium is a thin 

 cylindrical sheet of cells with very delicate walls, each 

 cell having the form of a rectangular prism with its ends 

 sharpened off like the cutting edge of a carpenter's chisel : 

 this prism is broader in the direction coinciding with the 

 plane of the sheet of cambium — i.e. in the tangential direc- 

 tion, with reference to the trunk of the tree— than in the 



Fig. 



Fig. s. 



Fig. 4. — Asmall block of wood from a sprace-fir, supposed to be magnified about loo times, showing elevation and sectional views of the tracheides of th^ 

 autumn (to the right) and spring wood, and medullary rays (m u) running radially between the tracheides. (After Hartig. ) 



Fig. 5. — Portion of cambium of a fir, showing the development of the young wood tracheides bom the cambium-cells. The arrow points to centre of the 

 stem. The cambium-cells at length cease to divide, and the walls become thicker (a), except at certain areas, where the bordered pits are developed 

 {6 and c). To the right is a medullary ray. Highly magnified, and the contents of the cambium-cells omitted for clearness. 



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 cir- 

 cumference of the cylinder enlarges. After a cambial 

 cell has extended 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 thiis 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 initiates the division : it 

 suffices to point out that by the above process of division 

 of the cambial cells there are formed radial rovv-5 of 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 numbers 

 towards the outside of the cambium cylinder {i.e. to add 

 to the cortex), but we are only concerned with the wood, 

 and therefore only regard those cells which are developed 

 on the inside {i.e. towards the centre of the stem). After 

 a time the oldest of these cells {i.e. those nearest the centre 

 of the stem) cease to divide, and undergo changes of 

 another kind : the process of division is still going on in 

 the younger ones, however ; and so the radial rows are 

 being e.^tended by additions of cells at their outer ends. 

 Of course, this is normally proceeding along the whole 

 area of the cylindrical sheet of cambium, and therefore 

 over the whole of the stem and roots, with their branches. 



