THE SPRUCE FIR 



245 



The reason why we can detect the annual rings is 

 that the wood formed in autumn has a different 

 structure from that developed in spring. Thus there 

 is a sharp boundary between the autumn wood of one 

 year and the spring wood of the next. This difference 

 in the case of the Fir-tree is due to the fact that in 

 the autumn wood the tracheides are much narrower 

 radially and have much thicker walls than those 

 formed in spring (see Fig. 100). The last-formed 



FIG. 100. Portion of transverse section through the wood of 

 Picea excelsa, showing spring and autumn wood. The 

 arrow c points towards the cambium, the arrow p to- 

 wards the pith ; a, autumn wood ; sp, spring wood ; a 1 , 

 beginning of autumn wood of second year. Magnified 

 330. (R. S.) 



layers of the autumn wood, unlike all others, have 

 pits on their tangential as well as on their radial 

 walls. Through these tangential pits the water 

 passes to the cambium, and to the spring wood as 

 soon as it is formed in the following year. 



The medullary rays vary very much in size. Many 

 of the rays are only one cell thick, and some of these 

 may be only one or two cells in height ; others are 

 several cells in thickness, and proportionately higher. 



