THE PALM-STEM. 21 



some of the component layers are often of a darker hue 

 than the rest. 



A second remarkable peculiarity of these cells is their 

 porosity. Both in the transverse and longitudinal sec- 

 tions we see fine striae, which run from the cavity toward 

 the outer surface of the cell. When a high magnifying 

 power is used, there remains no doubt that these striae 

 are canals perforating the cell-wall. As a general rule, 

 their diameter does not exceed ^~ of a line. As in the 

 case of the pores of cellular tissue, the canals of adja- 

 cent cells correspond to each other. 



The second constituent of the vascular bundle, which 

 I have termed wood, is composed of two organic systems, 

 cellular tissue and vessels. 



The cellular tissue of this woody portion consists of 

 colourless parenchymatous cells, the walls of which are 

 not very thick. They are usually somewhat elongated, 

 stand in vertical series one above another, with horizontal 

 septa ; never lie in series diverging like a fan from the 

 hindmost point of the woody mass, but form an irregular 

 parenchyma, the cells of which, in the vicinity of the 

 vessels, are arranged according to the form and position 

 of these latter. These cells never contain starch-granules ; 

 their walls are studded with large and small pores like 

 the cells of Cycas. 



The woody mass, as already mentioned, always lies at 

 the inner side of the vascular bundle ; but in the transi- 

 tion of the fibrous bundle, devoid of vessels, into the 

 condition of vascular bundle, it is very frequent, and 

 almost the rule, for the woody mass to lie, not at the 

 inner side, but in the middle of the liber-bundle. In the 

 vascular bundles of the outer, hard layers of the stem, 

 also, a narrow strip of liber-tubes often runs round the 

 posterior face of the woody mass, so that this is completely 

 surrounded by liber- tubes. 



In other cases the membrane of the wood-cells is itself 

 thickened, and thus they acquire a resemblance, at least 

 in the cross section, to the liber-cells ; however, they are 



