_500 SECONDARY CHANGES. 



uniform in the various species of these floating woods, as far as the existing de- 

 scriptions extendi. Hallier's figures (/. c.) and Figs. 51-53 in Schleiden's Grundzuge 

 (3 Ed.), I. p. 261, which at any rate represent a similar object, may serve to illustrate 

 its coarser characteristics. The following short description refers especially to the 

 wood of the Ambatsch of the White Nile (Herminiera Elaphroxylon=^demone 

 mirabilis Kotschy ^) : — 



The extremely light wood has no distinct annual rings. It consists, in its main mass, 

 of elements which must be called tracheides (until undried material has been investigated), 

 because in their condition as observed, they contain only air, without a trace of proto- 

 plasm or remnants of cell-contents. They stand alternately in radial rows, and have the 

 form of hexagonal upright prisms, about three times as high as broad, with terminal sur- 

 faces inclined to the radial plane at an angle of about 45°, either unilaterally, or bilaterally, 

 like a roof. Their thin colourless membrane is very beautifully thickened on the entire 

 terminal surface, by a fine delicate network of fibres, while on the radial, and in a lesser 

 degree on the tangential lateral surfaces, it is provided with small groups of simple 

 minute pits. 



The mass consisting of these tracheides is traversed (i) by very numerous parenchy- 

 matous medullary rays, containing starch, which are i-io, on the average about six cells 

 high and one cell in breadth, and further by some larger medullary rays, several cells broad 

 in the middle ; the cells of the medullary rays are elongated and procumbent. (2) By 

 narrow bands arranged in irregular and often interrupted concentric annular zones, 

 which are themselves crossed by the medullary rays; these bands chiefly consist of 

 long-pointed fibres, with their ends overlapping one another, in a radially and tangen- 

 tially oblique direction. In these bands, or more correctly on their inner (medullary) 

 side, lie large pitted vessels, usually isolated, rarely forming short radial rows of few 

 elements, and in both cases at long distances from one another laterally, being separated 

 by several subdivisions of the wood, as determined by the medullary rays. Every vessel, 

 or every group of vessels, is partly surrounded by a single layer of intermediate fibres 

 containing starch, or, by parenchymatous cells, with a moderately thickened pitted wall, 

 2-4 of which stand one above another ; and very thin-walled, narrow parenchymatous 

 cells, likewise containing starch, are continued in a single layer over the inner surface of 

 each fibrous band. Between these the above-mentioned (p. 141) chambered crystal-sacs 

 are here inserted. Tracheides, intermediate cells, members of the vessels, and also 

 the medullary rays of medium altitude, are everywhere of approximately equal height, 

 and lie with their ends in the same horizontal planes, thus forming regular horizontal 

 layers. Their form and arrangement (with the obvious exception of the members of 

 vessels) are similar to those usual in cambial cells, so that it may safely be assumed that 

 they are derived from a cambial zone consisting of cells similar to them in height and 

 form. The fibres, on the other hand, are (as estimated) at least double as long as the other 

 elements mentioned, and must thus have undergone the corresponding elongation (and 

 displacement) on their differentiation from the cambial zone. 



c. Changes of the individual forms of tissue in the annual ring. 



Sect. 151. In those of the cases just brought forward, in which the distribution of 

 the forms of tissue is different in the successive layers of an annual increment of growth, 

 and where the strdctm-e of the autumn-wood of one year is thus different from that 

 of the neighbouring spring-wood of the next year, a limiting line must appear between 



' Hallier, Botan. Zeitg. 1859, p. 152 ; 1864, p. 93. 



''■ Compare Schweinfurth, Beitr. z. Flora ^thiopensis, p. 9. 



