TISSUES OF PHANEROGA3I1A 



697 



cells (fig. 551). usually pointed at theiv two extremities so as to 

 become spindle-shaped, whose walls have a special tendency to 

 undergo consolidation by the internal deposit of sclerogen. It is 

 obvious that a tissue consisting of elongated cells, adherent together 

 by their entire length, and strengthened by internal deposit, must 

 possess much greater tenacity than any tissue in which the cells 

 depart but little from the primitive spherical form ; and we accord- 

 ingly find woody fibre present wherever it is requisite that the fabric 

 should possess not merely density, but the power of resistance to 

 tension. In the higher classes of the vegetable kingdom it consti- 

 tutes the chief part of the stem and branches, where these have a 

 firm and durable character ; and even in more temporary structures, 

 such as the herbaceous stems of annual plants, and the leaves and 

 flowers of almost every tribe, this tissue forms a more or less import- 

 ant constituent, being especially found in the neighbourhood of the 

 spiral vessels and ducts, to which it affords protection and support. 

 Hence the bundles of fasciculi composed of these elements, which 

 form the ' veins ' of leaves, and which give stringiiiess ' to various 

 esculent vegetable substances, are commonly known under the 

 name of Jibro-vascular tissue. In their young and unconsolidated 

 state the woody cells seem to conduct fluids with great facility in 

 the direction of their length ; and in the Coniferce, whose stems and 

 branches are destitute of ducts, they afford the sole channel for the 

 ascent of the sap. The Jibro-vascular bundles, which are the chief 

 strengthening elements of such organs as the stem, branches, leaf- 

 stalks, flower-stalks, etc., are. in the higher 

 plants, structures of considerable com- 

 plexity ; in Exogens they consist of three 

 distinct portions, the ccyfem-portion com- 

 posed chiefly of the different kinds of 

 vessels hereafter to be described, a pit loem- 

 portioii composed of proseiichymatous 

 tissue and ' sieve-tubes,' and a formative 

 eam&mm-portion. 



A peculiar set of markings seen on 

 the woody fibres of the Coniferce, and of 

 some other tribes, is represented in fig. 

 536 : in each of these spots the inner 

 circle appears to mark a deficiency of 

 the lining deposit, as in the pitted cells 

 of other plants ; whilst the outer circle 

 indicates the boundary of a lenticular 

 cavity which intervenes between the ad- 

 jacent cells at this point. There are 



varieties in this arrangement so charac- 



Fio. 536. Section of coniferous 

 wood in the direction of the 

 trache'ids, showing their 

 'bordered pits;' , </, a, me- 

 dullary rays crossing the 

 fibres. 



teristic of different tribes that it is 



sometimes possible to determine, by the 



microscopic inspection of a minute fragment, even of a fossil wood, 



the tribe to which it belonged. Markings of this kind, very 



characteristic of the wood of Coniferce, though not peculiar to that 



