THE PALM-STEM.

23

of the tubes do not lie one behind another in the direc-tion of a line drawn from the periphery to the centre ofthe stem, but side by side. The tubes do not alwayssucceed one another singly in the longitudinal arrange-ment, for frequently two or three stand next below asingle one, so that in the cross section there appear to be2—3 vessels side by side. This occurs sometimes evenlow down in the vascular bundle, in which case thesevessels often again blend into a single one. Some ofthese vessels attain a very considerable size. In thelower part of the course of the vascular bundle, theyhave indeed scarcely a diameter of ^ of a line; but thevessels occurring in the middle tract are among thelargest that are met with in the vegetable kingdom :thus the vessels of Badris mitis exhibit a diameter offrom L to i of a line, those of Desmoneus mitis, (Enocar-pus minor, 4$ to of Astrocaryum gynacantlmm toCorypha cerifera and Mauritia armata L to Mauritiavinifera 5 to 5, Calamus Draco 1 to 3 of a line.

The walls of these vessels universally exhibit the formof a dotted tube.* But the nature of this varies accordingto the nature of the adjacent parts. As this circumstancehas almost entirely escaped phytotomists, a more minuteexposition of it may not be out of place here. I havealready pointed out, that in cellular tissue the position ofthe pores of one cell exerts a definite influence over theposition of the pores of the adjacent cells, and that thepores of two contiguous cells always correspond. Now thisoccurs also in the reticulated vessels. It is a universallaw in the reticulated vessels and scalariform ducts of allplants, that the dots and slits of their walls are somewhatshorter than the breadth of the cell or vessel in contact

* In the Latin original I have called them vasa porosa, v. punctata,because the term reticulated vessel did not appear to me adapted to thekind of punetation. But since I have subsequently found that the pores ofdotted vessels of the Dicotyledons are distinguished from those of the vesselsof Palms, by the fact that there is a cavity between each pair of pores in theformer, which is wanting here, I now select the expression reticulated vessel,to avoid the necessity of making a new term.