. The microscope and its revelations. in it the portion of the cellular substanceknown as pith, whilst it is itself enclosed in an envelope of the samesubstance that forms the l>«rk. These two plans of axis-formationrespectively characteristic of those two great groups into \\hichPhanerogams are subdivided — namely, the Monocotyledons and theDicotyledons \\ill now be more particularly described. \Vlieu a iransv eise section (fig. .>:!K) of a monocotyledonous stemis examined is found to exhibit a number of iibro-vaseular bundles, disposed without anv regularity in the m
. The microscope and its revelations. in it the portion of the cellular substanceknown as pith, whilst it is itself enclosed in an envelope of the samesubstance that forms the l>«rk. These two plans of axis-formationrespectively characteristic of those two great groups into \\hichPhanerogams are subdivided — namely, the Monocotyledons and theDicotyledons \\ill now be more particularly described. \Vlieu a iransv eise section (fig. .>:!K) of a monocotyledonous stemis examined is found to exhibit a number of iibro-vaseular bundles, disposed without anv regularity in the midst ofthe mass of cellular tissue, which forms (as it were) the matrix orbasis of the fabric. Each bundle contains two. three, or more large STRUCTURE OF STEMS 701 ducts, which are at once distinguished by the size of their openings ;and these are surrounded by woody fibre and spiral vessels, thetransverse diameter of which is so extremely small that the portionof the bundles which they form is at once distinguished in transverse. FIG. 538.— Transverse section of stem of young palm. section by the closeness of its texture (fig. 539). The bundles are least numerous in the centre of the stem, and become gradually more crowded towards its circumference ; but it frequently happens that the portion of the area in which they are most compactly arranged is not abso- lutely at its exterior, this portion being itself surrounded by an investment composed of cellular tissue only ; and sometimes we find the central portion also completely destitute of tibro- vas- cular bundles ; so that a sort of indica- tion of the distinction between pith. wood, and bark is here presented. This distinction, however, is very im- perfect ; for we do not find either the central or the peripheral portions e\ in- separable, like pith and bark, from the intermediate woody layer. In its young state the centre of the stem is always filled up with cells ; but these not unfrequently disappear after a
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
