WOOD IN GENERAL 251 
the closed longitudinal tubes that form the main part of 
wood, cannot be all just alike for they occur in alternating 
layers of darker and lighter color. Examination of these 
layers under the magnifier shows that in the lighter colored 
layer the tubes are of decidedly larger bore than those form- 
ing the darker layer. The pale layers of less compact ma- 
terial are called spring wood, and the more compact layers, 
summer wood, for reasons that will presently appear. On 
a tangential surface, that is to say, one cut with the grain 
but not toward the center of the log, these contrasted layers 
her 
<a Be 
9 ane eal a; JIE 
He Det be 
Te PT é =f- 
HHA LO 
pty ing fate 
H a 
wae 
pee 
—asus 
oe 
— 
san 8 
= aoe 
Bid rttt 
a viigu 
CO a 
Fig. 230.—Tangential section of Fic. 231.—Transverse section of 
white pine wood, *°. (Original.) white pine wood, °°. (Original.) 
appear as broader bands often in beautiful systems of curves. 
On such a surface the cut ends of the pith-rays are to be 
seen under the magnifier (Fig. 230) as small, very narrow 
spots or streaks. If now we examine a crosscut or transverse 
section (7. e., a thin slice made at right angles to the direction 
of the fibrils) the magnifier will show us something more of 
the form and arrangement of the parts. As shown in Fig. 231 
we can look through the central cavities of the tubular fibrils, 
and so get a better idea of their sizes and shapes and the 
thinness of the walls. They are seen to be arranged in radial 
rows, between which the pith rays often appear as more or 
less delicate lines of dense material. Here and there among 
