127 
Xylem of Woody Stems . 
shaded away toward the middle lamella. The difference in the manner 
in which the fibre-walls reacted to chlor-zinc-iodine was most marked 
in the two cases, and this may perhaps be attributed to the different 
situations. 
In longitudinal sections these appearances are well-marked, the cellulose 
lining of the fibres being easily recognized. The cellulose ring round 
the pits on the walls of the vessels was exceedingly striking when stained 
in chlor-zinc-iodine. Fig. 4 is taken from the branch of the tree growing 
at the water-side. The iodine and sulphuric acid (76 per cent.) reaction 
agreed with the results obtained from chlor-zinc-iodine. In longitudinal 
sections, especially, it was most beautiful to see the edges of the pits 
turn blue in turn as the acid gradually swept over them. Further 
confirmation was obtained by treatment with Congo-red and also with 
haematoxylin. 
In the wood of Ulmus each annual ring commences with large wood- 
vessels succeeded by wood-parenchyma and fibres. When stained with 
haematoxylin and mounted in Canada balsam the wood-fibres are especially 
conspicuous by their thick yellow walls, inside which, in many instances, 
a distinct blue-violet lining of cellulose may be detected. 
An Alnus stem obtained from the banks of a stream in Warwickshire, 
about ten years old, and having a diameter of 2 cm., showed the presence 
of cellulose very distinctly round one half of the stem, while in the other 
half it was entirely absent. The development occurred chiefly in the third, 
fourth, fifth, and sixth years, being especially prevalent in the very wide 
fourth year, as indicated by the inner thickened layers staining clearly 
blue-violet with haematoxylin. 
A branch of Bettda six years old, with a diameter of 1.4 cm., was 
also examined. This, upon treatment with haematoxylin, indicated a 
special prevalence of the swollen cellulose layers in the fourth and fifth 
years. 
In Fraxinus , unlike the other woods examined, no well-marked patches 
of wood-fibres, in which a broad lining of cellulose is present, could be 
seen. The specimen from which my sections were taken was a vigorous 
and healthy sapling, with a diameter of 2.7 cm. and twelve annual rings. 
The stem had therefore grown quickly. It was obtained in July, and the 
next day sections were cut, stained with Delafield’s haematoxylin, and 
mounted in Canada balsam. In none of the sections, however, could any 
characteristic development of the thickening gelatinous layer be found ; 
at most on the internal surface of some of the fibres there might be 
detected a very thin line of blue, and this was found to be true of all 
the trees afterwards examined. 
It had now become perfectly plain that a thickening layer of cellulose 
occurred quite commonly as a natural feature in the woody fibres of 
