OF THE WOOD OF INDIAN SPECIES OF PINUS. . 477 
fraction of the annual ring varying from one-half to one-eighth. The 
medullary rays are visible in transverse seetion, especially in the summer- 
wood and darker heart-wood, as light-coloured lines, and in radial section as 
slightly darker narrow bands. Тһе resin-ducts are clearly visible and 
especially striking in longitudinal views as brown lines. They occur in the 
spring-wood and summer-wood, but appear to be more numerous in the 
middle and outer parts of the annual ring than in the inner. Their number 
appears to be inlependent of the relative amount of spring- and summer- 
wood. 
D. Micnoscoric STRUCTURE. 
Tracheids. 
The transverse section of the tracheids in the earliest spring-wood tends to 
be oblong rectangular with the long axis radial; further out this changes to 
square, and ultimately in the summer-wood to rectangular with the long 
axis tangential. The change in size of lumen and thickness of wall is shown 
in the following table. 
Width of Lumen, Mean thickness of Walls. 
Radial. | Tangential. Radial. Tangential. 
Early spring-tracheid .. 555p | 395 pu du ot р 
Late summer-tracheids . 105 u | 225p Sp 9°6 ш 
Typical length of tracheid 4 mm. 
Thus the outermost summer tracheids vie with fibres in thickness of wall 
and smallness of lumen. 
Our specimen regularly showed “double annual rings.” The spring- 
tracheids somewhere near the middle of the true annual ring rather abruptly 
give way to two or three layers of compressed tracheids with thicker walls 
and smaller lumina. Outside this belt the tracheids more or less completely 
assume the form of spring-tracheids with larger lumina and with walls 
either as thin as those in the spring-wood or considerably thicker. Outside 
this zone in turn more or less sudden thickening of the walls marks the 
beginning of the true summer-tracheids (fig. 18). 
The walls as seen in radial section widen markedly in contact with the 
medullary rays. 
The ends of the tracheids are more or less sharp, or bluntly rounded, 
except when they end abruptly on a medullary тау; when ending thus 
