WOOD OF PINUS. 145 



nucleus are now easy to see. It can be settled with equal 

 certainty that the trache'ides, when fully formed, lose all living 

 contents. The walls of the cambium, and the youngest adjoining 

 cells, have stained bright violet, those of the older portions of 

 the bast a dark violet colour. The contents of the crystallo- 

 genous cells remain brown, while those of the cells of the cork- 

 layer now appear reddish-brown. The specially thin inner walls 

 of the cells surrounding the resin-canal, i.e., those actually 

 forming the resin-passage, usually stain a dirty violet. Careful 

 examination shows, moreover, that the closing membrane of the 

 unilaterally-bordered pits has stained violet, while that of the 

 bilaterally-bordered pits remains uncoloured. Starch, wherever 

 present, takes the iodine reaction. 



In Chap. V., p. 78, we have already seen the Lignin reactions 

 displayed by coniferous wood. Corallin also, in virtue of pro- 

 perties already known to us, stains the lignified cells quite 

 differently from those which are unlignified. We obtain, indeed, 

 very beautiful and instructive preparations when the sections are 

 laid for some time in soda-corallin and then examined in glycerine. 

 The lignitied membranes are stained a deep red ; towards the 

 cambium this red disappears, and passes over into pale yellow. 

 In the bast the cell-walls have a pale reddish-yellow coloration ; 

 the sieve-plates are coloured deep red, especially where they are 

 covered with a callus-layer. As corallin also stains starch grains 

 rose-colour, these consequently stand out quite sharply in the 

 outer parts of the bast. 



We will now prepare some radial lomjitiuLiiial sections, again 

 using the alcohol material. 



Structure of the Wood. Such a section shows us in the wood 

 the trache'ides with bordered pits. The ends of the trache'ides 

 interlock. In the spring trache'ides these ends are rounded ; in 

 the late summer tracheides they are pointed. The bordered 

 pits in face view show the form of two concentric circles (Fig. 27 

 A). The inner small circle, or (occasionally) ellipse, indicates the 

 opening of the pit into the cavity of the tracheide ; the large 

 outer circle indicates the broadest part of the pit, that which is 

 bounded by the primitive wall separating the two trache'ides. It 

 is this primitive wall which, thickened at its middle into the 

 torus, constitutes the closing membrane. The torus can often 

 be distinguished as a faintly shining disk of about twice the 

 diameter of the mouth of the pit. Especially in sections of dry 



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