408 KANSAS ACADEMY OF SCIENCE. 



off at maturity ; or at the end of a year's growth in the twigs of trees, 

 as in Cottonwood or elm. Except in the main branches of such trees 

 the cells often continue for a few years to end at the same place, the 

 joint or ring. Eventually, if the twig be not shed with the leaves, 

 the cells lap over and the ring disappears. 



STRUCTURE OF EVERGREEN TREES. 



In all the evergreens ( conifers ) the cells are comparatively large, 

 uniform, and arranged in remarkably straight rows radially ; the radial 

 rows of cells are very close together, making all cell-walls thin. In 

 many species the wood cells are formed at all seasons of the year with 

 about the same rapidity ; yet the wood of winter is decidedly harder, 

 more compact, and of a higher color than that of spring and summer, 

 thus making the rings quite as easily counted as in deciduous trees, 

 or those having annual leaves. The resin ducts are much larger than 

 the cells, being from 80 to 160 microns in diameter; are scattered 

 and irregular, and most generally in the outer though sometimes in 

 the inner part of the rings. The radial plates or medullary rays are 

 very narrow and thin, consisting of two to fifteen or more horizontal 

 parallelepipedal cells radially extended and laid in a single series ver- 

 tically, like bricks set on edge lengthwise in a wall, rarely two or 

 three series together, and extending from the jDith or from some place in 

 the interior of the tree to and sometimes slightly into the bark. These 

 cells carry food from the elaborated sap in the space between the 

 wood and bark and from the inner layers of bark to the interior of 

 the tree, to feed and strengthen the cells previously formed. They 

 also strengthen the wood of the tree by crossing the regular grain and 

 binding the rings. 



In megacellular trees, such as common hemlock, bald cypress, noble 

 fir, coast redwood, and sugar pine, which have the largest cells of any 

 of the evergreen trees, the cells measure thirty-five to forty-five 

 microns in cross section tangentially to the tree, and from thirty to 

 fifty microns radially ; and there are from 300,000 to 600,000 cells to 

 the square inch. These can be readily seen, but not readily counted, 

 with the unaided eye. 



In mesocellular trees, such as giant redwood, Monterey cypress, 

 most of the firs, the spruces, and nearly all the pines, the cells meas- 

 ure twenty-five to thirty-five microns tangentially, and twenty to forty 

 microns radially ; and are 600,000 to 1,200,000 cells to the square inch. 

 These cannot be counted without a lens. 



In microcellular trees, such as mountain hemlock. Pacific coast 

 cypress, California red and white fir. Pacific yew, and all the cedars 

 and junipers, the cells measure fourteen to twenty-eight microns in 

 diameter either way ; and are 1,200,000 to 2,500,000 cells to the square 



