BORDERED PITS. 77 



pits of the air-dry wood it is usually pressed to one side of the 

 pit-chamber (B). If, on the other hand, fresh wood, or alcohol 

 material, is investigated, we shall find the closing membrane in 

 the sap-wood (alburnum) stretched across the middle of the pit- 

 chamber. In the heart- wood (duramen), on the contrary, the 

 relations are just as we have given for the air-dry wood. 



The figure of the bordered pit is clearer after the action of 

 chlorzinc iodine, which stains the cell-wall yellow-brown. This 

 coloration is due to the strong lignif action of the walls. Here 

 and there these walls appear bordered with violet on their inner 

 sides. The closing membrane is not stained by the chlorzinc 

 iodine. After treatment with this reagent we can readily con- 

 vince ourselves that the completely- developed wood-cells contain 

 neither protoplasmic sac nor nucleus ; they consist only of dead 

 cell -walls, and, as they serve for the conduction of water, and in 

 this respect, as well also as in the nature of the thickening of 

 their walls, resemble the tracheae, i.e., vessels, they are known as 

 tracheides. 



Not infrequently the thickening layers of the tracheides of the 

 pine-wood, when examined in longitudinal section ; show a more 

 or less distinct spiral striation, mounting at an angle of about 50 

 to 60. This arises from the formation of narrow spiral bands, 

 instead of an uninterrupted thickening layer. The mouths of the 

 pits then appear elongated in the direction of the striation, and 

 like the striae of the two adjoining side- walls, the mouths of 

 adjoining pits cross one another. 



We prepare also a cross-section of the pine-wood. This must 

 be specially thin. The tracheides thus cut across appear as a 

 rule rectangular. They form radial rows. The limits of the 

 annual rings are marked out by the abrupt succession of the wide 

 cavitied slightly thickened tracheides of spring, to the narrow 

 cavitied greatly thickened tracheides of late summer. Each 

 medullary ray is formed only of a row of narrow, radially 

 elongated cells. On the radial walls of the tracheides we see 

 the sections of the pits (Fig. 27, C), the figure of which appears 

 in no way different from that in the tangential longitudinal 

 section. Between the tracheides the middle lamellae appear as 

 fine separating lines (m). Where more than two tracheides are 

 in contact, the middle lamella is thickened into a solid or hollow 

 "seam" (m*). The inner limit of the cell-wall surrounding the 

 cavity is more strongly refractive, and forms the limiting mem- 



