368 



THE MICROSCOPE AND ITS REVELATIONS. 



are at once distinguished by the size of their openings; and these are 

 surrounded by woody fibre and spiral vessels, the transverse diameter of 

 which is so extremely small, that the portion of the bundles which they 

 form is at once distinguished in transverse section by the closeness of its 

 texture (Fig. 251). The bundles are least numerous in the centre of the 

 stem, and become gradually more approximated to wards its circumference 

 but it frequently happens that the portion of the area in which they are 

 most compactly arranged is not absolutely 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 

 fibro-vascular bundles; so thab a sort of indication of the distinction be- 

 tween Pith, Wood, and Bark is here presented. This distinction, however, 

 is very imperfect; for we do not find either the central or the peripheral 

 portions ever 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 time, except at the- 

 nodes, leaving the stem hollow, as we see in the whole tribe of Grasses. 



Diagram of the first formation of Transverse Section of Stem of Clematis: a. pith; 6, 6, &,. 

 an Exogenous Stem ; a, Pith; 6 fo, woody bundels; c, c, c, medullary rays. 



Bark; c c, plates of cellular tissue 

 (Medullar Kays) left between the 

 Woody Bundles d d 



When a vertical section is made of a woody stem (as that of a Palm) of 

 sufficient length to trace the whole extent of the fibro-vascular bundles, 

 it is found that whilst they pass at their upper extremity into the leaves, 

 they pass at the lower end toward the surface of the stem, and assist, by 

 their interlacement with the outer bundles, in forming that extremely 

 tough investment which the lower ends of these stems present. New 

 fibro-vascular bundles are being continually formed in the upper part of 

 the stem, in continuity with the leaves which are successively put forth at 

 its summit; but while these take part in the elongation of the stem, they 

 contribute but little to the increase of its diameter. For those which are 

 most recently formed only pass into the centre of the stem during the 

 higher part of their course, and usually make their way again to its ex- 

 terior at no great distance below; and when once formed, they receive 

 no further additions. It was from the idea formerly entertained that 

 these successively-formed bundles descend in the interior of the stem 

 through its entire length until they reach the roots, and that the stem is 

 thus continually receiving additions to its interior, that the term endoge- 

 nous was given to this type of stem -structure; but from the fact just 

 stated regarding the course of the fibro-vascular bundles, it is obvious 

 that such a doctrine cannot be any longer admitted. 



