384 



Vegetable Physiology. 



Fis. 7. 



many kinds of wood, so that the originally tubular cells become 

 almost solid cylinders, in which state they are sometimes called 

 woody fibres (fig. 7). The liber-cells, forming the woody struc- 

 tures of bark, are much 

 longer in proportion to 

 their diameter than the 

 cells of wood, and from 

 their great solidity when 

 fully developed are often 

 called ' liber-fibres.' The 

 wood -cells, and also the 

 liber-cells to less extent, 

 have their walls marked, by 

 the absence of the thicken- 

 ing layers at certain spots, 

 and a great variety of condi- 



Cross section of wood of the rScotch fir, showing the ends ,• ( tV, t 'f ' 1 * 



of a number of wood-cells. The section having been tions OI lliese pilS Or ClOtS 

 heated in nitric acid tliey are partially separated, and ^ in the WOods of dif- 



the concentric lamellai of their walls have become 



visible. Magnified 4uu diameters. ferent trees ; It is altogether 



unimportant for our purpose 

 to dwell upon these. It may be mentioned, however, that the wood 

 of most Dicotyledons contains, scattered in the substance of the 

 bundles, large canals formed by perpendicular cells of cylindrical 

 form coalescing at the ends, so as to form a continuous tube. 

 The spiral and annular vessels of fully developed bundles coalesce 

 in a similar way at their ends, and the jointed tubes formed in 

 this way are often called ' ducts.' These ' ducts ' are often so 

 large that they constitute tubular channels visible to the naked 

 eye, as may be seen in the wood of oak, mahogany, &c., and like- 

 wise in the Monocotyledonous cane, the stem of a kind of palm. 

 The use of these ducts is unknown ; in ordinary conditions they 

 contain air, and perhaps they may be regarded as contrivances to 

 lighten the woody structure, and to serve at the same time as 

 ' safety-tubes ' into which the fluid contents of adjoining cells 

 may be poured out when the tissue is gorged with sap. 



Diverse as we find the forms and the physical conditions of the 

 cell-membranes and their thickening layers in the various tissues 

 of the higher plants, they have a fundamental identity of compo- 

 sition. In all cases they are originally composed of cellulose, a 

 substance having a definite chemical constitution, and which is 

 recognizable by certain characters of reaction which can be applied 

 to the tissues under the microscope. The analyses of cellulose 

 show that it is most intimately related to starch, and it is well 

 known that starch acquires a blue colour when it is brought in 

 contact with iodine, a fact which is of great value to the vegetable 

 anatomist in enablinir him to detect the existence of starch in the 



