^8 MORPHOLOGY OF TISSUES. 



year is similar to that already described. In Fig. 8i, for example, the annular 

 vessel r is first formed in the xylem portion, then the spiral vessel s, then advancing 

 right and left the pitted vessels g g, and in the middle (advancing radially) the 

 narrow pitted vessels. It sometimes occurs {e. g. in Calodracon, according to 

 Nageli) that the formation of vessels advancing right and left encloses the pro- 

 cambium, which afterwards passes over into latticed cells. In the leaf-stalk of 

 Pteris aquilina the development of the xylem begins in the procambium bundles 

 with an elliptical transverse section, by the formation of some narrow spiral vessels 

 in the foci of the section ; then, following the longer axis, scalariform vessels 

 are formed, first centrifugally then centripetally, until a compact woody mass is 

 produced, elongated in transverse section ; around this the still remaining pro- 

 cambium is transformed into latticed cells, sieve-tubes, and cambiform tissue, and 

 partly (at the circumference) into bast-fibres (Figs. 83, 87, A). 



The fibro-vascular bundles of roots arise in a tissue which is differentiated 

 out of the primary meristem of the apex of the root in the form of a solid (rarely 

 hollow) cylinder. In this the development of vessels begins at two, three, four, 

 or more points of the circumference, and advances radially inwards. If the pro- 

 cambium is a solid cylinder, a diametral row of vessels (seen in transverse section) 

 arises, or a star of three, four, five, or more rays, the youngest and broadest vessels 

 lying next the axis ; between the starting-points of this formation of vessels bundles 

 of phloem generally arise, and in roots which increase in thickness subsequently 

 cambial tissue, which is then developed in centrifugal order, as occurs in the stem, 

 vessels, and wood-cells ^ 



Forms of Cells. In the text I have indicated only the relative positions of the 

 separate forms of tissue of the fibro-vascular bundle in their most important features ; 

 some remarks will naturally follow on the forms of their cells ; but here also, in conse- 

 quence 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 the corresponding phenomena in 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) Fibrous, and (3) Parenchy- 

 matous. 



To the Vascular forms belong the ducts and the vascular wood-cells or Tra- 

 cheides. This group of forms is characterised by their walls forming open orifices 

 where two cells of the same form meet, so that their cell-contents soon disappear 

 and air takes their place ; the thickenings show a tendency towards the formation of 

 spiral bands, net-work, and bordered pits. True vessels (Figs. 27, 84) arise when the 

 septa of cells whose form is similar, arranged in rows longitudinally over one another, 

 are entirely or partially absorbed ; and thus long air-conducting tubes originate, 

 consisting of many cells, distinguished from the adjoining wood-cells principally by 

 their greater breadth. The septa may be placed horizontally or more or less ob- 

 liquely ; and in general the mode of their perforation is directed accordingly ; hori- 



^ Cf. Van Tieghem, Recherches sur la symmetrie de structure des plantes vasculaires, Paris 1871 



