128 



MORPHOLOGV OF TISSUES. 



course of these bundles necessitates that they must cross one another, and form 

 a loose mass consisting of slender isolated strings, surrounded by a more or less 

 thick layer of denser woody substance. This woody substance forms a cylinder 

 which, in decayed stems, is altogether separated from the layer of periderm, and 

 loosely enveloped by it. The isolated strings in the interior are the primary 

 vascular bundles which have been formed during the growth in length (properly only 

 their lower ends or Leaf-traces, since the upper ends bend outwards into the 

 leaves). The woody cylinder which envelopes them all consists, on the contrary, of 



secondary fibro-vascular bundles formed by 

 the increase in thickness, which are closely 

 crowded and anastomose copiously with 

 one another both in the tangential and 

 radial directions, and thus form, accord- 

 ing to circumstances, a more or less com- 

 pact or spongy mass, the true nature of 

 which it is easy to recognise in Aloe 

 and Beaucarnea. The course of develop- 

 ment of these stems is as follows. The 

 isolated fibro-vascular bundles (which in 

 old specimens are found in the interior) 

 are formed in the primary meristem of the 

 apex of the stem, while the whole remaining 

 tissue between them passes over into pri- 

 mary fundamental tissue ; but after con- 

 siderable time (in Aletris fragrans it takes 

 place about 4 or 5 cm., in Draccena reflexa 

 as much as 17 to 20 cm. below the apex 

 of the stem) a fresh formation of (second- 

 ary) meristem begins in one of the cell- 

 layers of the fundamental tissue which 

 immediately surround the outermost fibro- 

 vascular bundles. The permanent cells 

 concerned in it divide repeatedly by tangen- 

 tial and subsequently sometimes by radial 

 walls ; and there arises (seen in transverse 

 section) a ring of meristem (Fig. 104, x), 

 the cells of which are arranged in radial 

 rows. In this meristem new fibro-vascular bundles are produced ; one, two, or more 

 adjoining cells (on the transverse section) dividing repeatedly by longitudinal walls 

 in various positions. Out of the procambium-bundles which arise in this manner 

 the fibro-vascular bundles proceed immediately, the procambium-cells being trans- 

 formed into fibro-vascular tissue^; the intermediate meristem passes over likewise 



Fig. 104.. — Part of the transverse section of a stem oi Dra- 

 caua (probably reflexa) about 13 mm. thick and 1 metre high, 

 about 20 cm. below the summit, e epidermis ; k cork (peri- 

 derm) ; r cortical portion of the fundamental tissue ; /' transverse 

 section of a fibro-vascular bundle, bendinar out to a leaf; m the 

 primary fundamental tissue (pith) ; g the primary vascular 

 bundles ; x the ring of meristem in which very young fibro- 

 vascular bundles are to be seen, while the older ones g have 

 already partially or entirely passed out of it, its lower part 

 becoming transformed into fundamental tissue st arranged 

 in radial rows. ' 



^ It appears, however, that the thick-walled lignified cells on the outside of such a bundle 

 do not belong to it, but to the secondary fundamental tissue, and therefore represent only a 

 sclerenchymatous bundle-sheath, while the bundles enveloped by them are themselves very slender. 



