152 General Botany 



before the underground stems become large enough to send up 

 thick, upright branches suitable for marketing. 



The increase in thickness of stems of the palm type has already 

 been described. Usually the growth in thickness ceases after a 

 time, and the further growth of the stem takes place only in the 

 terminal bud. Such stems taper for a short distance at the base, 

 but above they are quite cylindrical. 



Annual rings. The wood derived from conifers and dicots have 

 certain characteristics which are due to the variation in growth 

 during spring and summer. It is the difference in size of cells 

 and thickness of walls laid down at different times of the year 

 that make the annual rings visible. In such woods as the oak, 

 ash, and yellow pine the annual rings are conspicuous because of 

 the difference in texture of the spring and summer wood. Beech, 

 birch, and redwood have quite inconspicuous annual rings because 

 growth is quite uniform throughout the growing season. In all 

 trees, however, there is a perceptible slowing down toward the 

 end of summer and the wood cells are smaller near the outer edge 

 of a ring. 



The width of rings is primarily dependent upon the amount of 

 carbohydrate furnished by the leaves, and secondarily on the 

 water supply. In wet seasons the rings are wider, in dry seasons 

 narrower. Indeed, the width of annual rings shows such perfect 

 correlation with wet and dry years, that the rings of our oldest 

 trees are being measured and studied to determine periods of 

 excessive rainfall and drought during prehistoric times, and to 

 estimate changes in climate. 



Classification of woods. The structure of the wood of any 

 species of tree is so characteristic that any piece of wood may 

 be identified by a careful examination. Woods are primarily 

 classified as ring porous, diffuse porous, and non-porous. The 

 " pores " refer to the openings among the wood cells made by the 

 tracheae or water tubes. Since these are not present in conifers, 

 conifer woods all belong to the third class. 



