ej . , SECONDARY CHANGES. 



the sieve-tubes (corap. p. 173)- It may here not be superfluous to add, that, so 

 far as my experience extends, the same statement also applies essentially to the 

 common officinal barks, and that it is only the dried condition in which they 

 usually come under observation which has hitherto, to some extent, impeded the 

 clear recognition of the actual conditions: 



The elements of the soft bast in woody plants of the families Apocyneae (Nerium 

 Oleander), Asclepiadeffi (Asclepias curassavica), Convolvulacese (Convolvulus Cne- 

 orum), and Campanulaceje (Campanula Vidalii); are less regularly arranged than in 

 the cases hitherto described, and this is no doubt also the case in the Cichoriace», 

 and in the plants mentioned at p. 324 (2), which are characterised by very small sieve- 

 tubes in the primary bundles. Between relatively wide parenchymatous cells, which in 

 some degree maintain the serial arrangement, groups of narrow elements are found 

 in these instances, which, as seen in cross-section, show very various, triangular, or 

 polygonal forms, and have apparently arisen from repeated longitudinal divisions 

 taking place in every direction in the original mother-cells of the tissue. The narrow 

 elements which are thus grouped are the sieve-tubes and cambiform cells; in the 

 Cichoriacese, Campanula, and Lobelia, the laticiferous tubes also occur in connection 

 with the groups of narrow elements. 



The arrangement of the elements of the bundle in the soft-bast, as just described, 

 prevails both in the stems and roots of woody plants, so far as the existing investi- 

 gations extend. 



There have been few minute investigations on the stems of herbaceous plants, 

 but, according to the existing observations, they are not essentially different frorn 

 woody plants with reference to the conditions here in question. 



Those roots which consist chiefly of parenchyma, with a very thick secondary bast, 

 as described above at p- 516, show in the latter a dislribution of the sieve-tubes 

 which' is sirnilar to that of the vessels in the massive parenchymatous wood of the 

 root. A Strand of bast corresponds to every ligneous strand, and the former contains 

 relatively small groups of narrow sieve-tubes, accompanied by narrow elongated 

 cells, and enclosed in a mass of large-celled parenchyma. ' 



The more special distribution, relative amount, and form of the tissues under con- 

 sideration is very various, according to the particular case, and often even in closely 

 related plants. In most cases the bundles, as seen in cross-section, form relatively 

 narrow radial bands, lying in the same straight line with the ligneous strands, between 

 rays of wide-celled parenchyma, which essentially, though not always quite exactly, form 

 the continuation of those of the wood. The strands consist of narrow, elongated cells, 

 which may even be pointed like a spindle (the latter, e.g. in the wild form of Daucus 

 Carota), and of sieve-tubes lying between them, which are always scantily developed, and 

 are likewise narrow. The individual radial bands, like the ligneous strands, are either 

 continuous, or are more or less interrupted by interpolated large-celled parenchyma. 

 Examples of this arrangement are afforded by many roots of Umbelliferse, Scorzonera 

 hispanica, Cichorium, Argemone, &c. 



In the roots of Rhubarb (Rheum undulatum and Rhaponticum) essentially the same 

 distribution reappears, but is modified by the forms and relative numbers of the 

 histological elements under consideration. The uniseriate, procumbent, parenchymatous 

 rays of the wood are continued without interruption through the zone of bast. The 

 strands separated by them consist, as regards much the greater portion of their mass, 

 of large, upright^ parenchymatous cells, filled with starch, and between these lie the 



