it6 
MORPHOLOGY OF TISSUES. 
primary roots of seedling Dicotyledons and Conifers, are thick in their upper part, 
gradually decreasing in thickness below as they lengthen, have no pith in their lower 
part, so that the vascular bundles, separated above by this pith, are in contact in the 
lower parts of the root, presenting a star-shaped appearance on transverse section. 
The outermost portion of the axial cylinder consists (except, according to Nägeli 
and Leitgeb, in the case of Equisetum) of a simple layer of cells, which they have termed 
Pericambium. It must lie, as will be seen from what has been said, on the inside of the 
plerome- or bundle-sheath (Fig. <)6, pc). The importance of the pericambium in the 
branching of the root and in its increase in thickness will be discussed further on 
in Sects. i8 and 23. 
In the present state of our knowledge of tissues it is still doubtful whether the axial 
cylinder of roots should be regarded as a single fibro-vascular bundle, or as a coalescence 
of a number of such bundles, corresponding to the bundles of xylem and phloem. Van 
Tieghem (/. c.) adopts the latter view, laying special stress on the fact that in most stems 
also a plerome-sheath (which he terms ' membrane protectrice ') separates all the fibro- 
vascular bundles together with the pith from the cortex. On this view it is only 
necessary that the bundles in the stem should be closely crowded together, and their 
phloem-portion placed at the side of instead of in front of the xylem-portion, in order 
to get an axial cylinder like that of roots. This view is supported by the fact that 
in some stems (as in those of Lycopodium, Fig. 100, B. p. 122, and some water-plants like 
H'ppuris, Hydrilla, &c.) there is an actual axial cylinder similar to that of roots. 
Further investigation is, however, required to show how far one is justified in regarding 
the entire axial cylinder of roots, like that of the stems just referred to, as the result 
of a longitudinal coalescence of true fibro-vascular bundles. 
Forms of Cells. I have at present indicated only the relative positions of the 
separate forms of tissue in the fibro-vascular bundle ; some remarks will follow on the 
forms of their cells ; but here also, in consequence of the numerous special modes of 
development, reference must be made to the special morphology of separate classes 
of plants in Book II. The cell-forms of the fibro-vascular bundles attain their most 
perfect and varied development in Dicotyledons ; the forms which occur in them may 
therefore be employed as a basis for the critical examination of those of other classes of 
plants. 
The Xylem-portion of the fibro-vascular bundle of Dicotyledons is composed of 
numerous cell-forms, which may be referred, according to Sanio's careful researches, 
to three types. He distinguishes (i) Vascular, (2) Prosenchymatous, and (3) Paren- 
chymatous. 
To the Vascular forms belong the Ducts (pitted or dotted wood-vessels) and the 
vascular wood-cells or Tracheides. This form of cells is characterised by their walls 
forming open orifices where two cells of the same kind meet, so that their contents soon 
disappear and air takes their place ; the thickenings show a tendency towards the 
formation of spiral bands, reticulations, and bordered pits. True Vessels (Figs. 25, 94, pp. 
26, 113) arise when the septa of similar cells arranged in longitudinal rows are entirely or 
partially absorbed ; and thus long tubes originate, filled with air, and distinguished from the 
adjoining wood-cells principally by their greater breadth^. The septa may be horizontal 
or more or less oblique ; and in general the mode of their perforation corresponds ; 
horizontal walls are often entirely absorbed, or are pierced by large round openings. The 
more oblique the septum, the more do the perforations take the form of narrower or 
broader parallel fissures ; and the thickening-bands of the septum which remain present 
more or less the appearance of rungs of a ladder, while they often combine into a net- 
work. These scalariform septa are found, according to Sanio, not only in reticulately 
thickened vessels and those with bordered pits, as was previously supposed, but also in 
spiral vessels {e.g. in Casuarina, Olea, Vitts), where coils of the spiral band pass imme- 
^ See, however, p. 90. 
