388 PRIMARY ARRANGEMENT OF TISSUES. 



As regards the special structure, the general rule holds good, that at the points 

 of attachment of lateral organs on stems and roots the elements of the vascular 

 bundle are short in comparison with those of the main bundles, because they in some 

 cases arise at such places as never undergo more than a slight elongation, and in 

 others attain their development after the elongation of that portion of the main axis 

 which bears them is far advanced. This phenomenon is especially familiar in the 

 case of the nodes of vascular plants ; it appears the more conspicuously the more 

 abundant the subdivision and ramification of the bundles where they are attached 

 (comp. Sects. 94, 95). The lateral walls of the short joints of vessels or of tracheides 

 in these regions are in the very great majority of cases pitted, or reticulately thick- 

 ened, and in the latter case they usually have low, transversely elongated meshes. 

 Spiral and annular threads seldom occur, and then usually pass over within a short 

 distance into cross-meshed reticulate thickening, e. g. in the node of many Comme- 

 lineae. The tracheal elements of one bundle as a rule attach themselves to the equi- 

 valent elements of another with tapered, acute ends, which adhere for some distance 

 to the lateral wall of the other ; the attached ends are rarely cut off transversely. 



The attachment of the sieve-tubes to one another appears to take place in a 

 similar manner to that of the vessels, especially according to isolated observations on 

 the Cucurbitacece. IMore accurate investigadons on this point do not however exist. 



C. DEVELOPMENT. 



Sect. 115. The vascular bundle is formed from a strand of meristematic cells, 

 which, in accordance with the definitive form of the bundle-elements, assume a form 

 elongated in the longitudinal direction of the bundle, and undergo divisions in the 

 same direction excepting where they give rise to the shorter parenchymatous 

 cells belonging to the bundle. These initial strands of the bundles are therefore dis- 

 tinguished, in a degree corresponding to the progress of histological differentiation, 

 from the surrounding non-equivalent tissue, and especially from the initial layers 

 of those masses of parenchyma which continue short-celled owing to persistent 

 transverse divisions. To this is further added the usually smaller growth of the ele- 

 ments of the bundle in a transverse direction, as compared with the short-celled 

 surrounding tissue ; the inidal strand has narrower cells than the surrounding tissue. 

 Essentially the same phenom.enon appears when other strands consisting of elongated 

 elements, especially for example strands of sclerenchymatous fibres, become differen- 

 tiated out of the primary meristem, and from tissue which remains short-celled, what- 

 ever may be the anatomical definition of the region in which they arise. Therefore 

 where a fibrous-strand immediately accompanies a vascular bundle, it is not to be dis- 

 tinguished from the latter in the initial stage, or at least not sharply. 



The long-celled initial strands of the vascular bundles, and in certain cases 

 the fibrous tissue accompanying them, were called by Nageli^ Cambium-strands, in 

 partial agreement with the older terminology, the term Cambium being used for 

 the long-celled initial strands, as distinguished from the short-celled 'Meristem.' 



* Beitr. I. p. 2. Compare the footnote at p. 4. 



