In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.
The gaps in the outer tubular stele, however, are formed by the departure of aerial branch-traces, instead of leaf-traces as in the ferns.
The result is, on the one hand, a clearing away of much fantastic phylogeny, on the other, an enormous reduction of the supposed gaps between groups.
The increasing number of measurements of the height of land in all continents and islands, and the very detailed levellings in those countries which have been thoroughly surveyed, enable the average elevation of the land above sea-level to be fairly estimated, although many vast gaps in accurate knowledge remain, and the estimate is not an exact one.
It is perhaps noteworthy that Xavier himself never mentions Pinto; but the omission may be explained by the numerous gaps in his correspondence.
