390 



PRIMARr ARRANGEMENT OF TISSUES. 



while growth in the transverse direction is relatively slight. These divisions are 

 repeated with a frequency varying according to the particular case. In thick and 

 especially in collateral bundles, they often still go on for a long time on the boundary 

 between phloem and xylem, when at the edges of the bundle the differentiation of the 

 tissues is already completed. In this case the later-formed septa in the border-zone 

 just mentioned assume the tangential direction spoken of above (p. 325); the earlier 

 stages of division always take place in all directions successively. 



The completion of the definitive elements of the bundle begins m each trans- 

 verse section with those of them 

 which have been described above ■ 

 as the primitive elements, and 

 thus in the regions occupied by 

 the latter. It spreads centri- 

 fugally from each of these over 

 the cross-section, when they are 

 situated in the interior of the 

 strand. If they occupy one edge 

 of the strand the development in 

 general proceeds towards the op- 

 posite edge; in radial bundles, 

 as described above, it goes on in 

 the centripetal. direction; in col- 

 lateral bundles it is centripetal 

 in the phloem and centrifugal 

 in the xylem ; in concentric bun- 

 dles, and in the special case of 

 leaves of Cycads described above 

 at p. 336, the development goes on 

 in different directions according 

 to the number and position of 

 the primitive groups, the direction being generally determined by the rule first 

 stated. Where the edge of the phloem is broad the development extends along 

 it from the first points of development in the same order. The development always 

 begins with that of the first primitive elements of the phloem, Russow's protO' 

 phloem ; the primitive elements of the xylem appear later in course of development. 

 Where a bundle is accompanied by sclerenchymatous fibres the development of 

 the latter takes place later, often much later, than ,that of the two primitive groups, 

 and independently of their appearance. 



With reference to the longitudinal course of the bundle, both the definitive 

 completion and the first origination may proceed in the acropetal, or conversely in 

 the basipetal direction K It is further at least conceivable that in the same bundle the 

 progression of the two processes may take place in unlike directions, or even that 

 each of them goes on in a different direction at different parts of the course of the 

 bundle. Sanio's statement ' that, according to his investigations,^ carried out in thes 



Fig, z?6 (aio), — Polygonum Fagopyrum. Root-apex, median longitudinal 

 section, pc pericambium, outside boundary of the plerome strand ; v riidiment 

 of a vessel ; € dermatogen or epidermis ; between ^c and e periblem ; h root- 

 cap. 



* [Compare Trecul, Ann. Sci. Nat. 6 ser. torn. XII. p. 251, 1881.] ^ Bptan, Zeitg. 1864, p. 194. 



