SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 35 



Precisely similar to the growth in thickness in Phaseolus is that of the primary 

 roots containing pith of seedlings of Cucurbita Fepo, Convolvulus tricolor, Cereus, 

 Clusia, &c. ; only that in these cases the secondary xylem does not coalesce with 

 the primary xylem, but remains quite distinct, so that the alternation of the primary 

 and secondary elements of the xylem is at once evident \ In slender roots which 

 have no pith, and where the primary xylem-bundles meet in the centre, cambium 

 is also formed on the inner side of the primary phloem, and the secondary fibro- 

 vascular masses form, therefore, in this case also two, three, or more groups, 

 which originate in the intervals between the primary xylem-bundles, as is very 

 clearly seen in Tropceolum, but project much further outwardly. When there are two 

 of these primary xylem-bundles, as in Beta, Tropceolu?n, Taxus, and Umbelliferae, 

 they form a vascular band dividing the axial cylinder in half; when three or more, 

 as in Pisum, a three- or four-rayed star. 



In the cases hitherto considered, the secondary fibro-vascular tissues (consisting 

 of xylem and the phloem belonging to it) remain separated into two, three, four, 

 or more masses, nothing but parenchymatous fun- 

 damental tissue being formed between them and 

 in front of the primary xylem, as in Phaseolus. In 

 other cases, on the contrary, true cambium is formed 

 in front of the primary xylem, producing xylem on 

 the inside, phloem on the outside \ and thus is 

 formed a compact cylinder of secondary xylem., 

 surrounded by a continuous layer of secondary 

 phloem, as in Taxus, Pinus, Beta, &c., and as is 

 shown, in the case of the stem, in Fig. 105. 



The secondary xylem of roots very frequently 

 consists for the most part of succulent wood- 

 parenchyma, in which the few vessels, surrounded fig. lOj.-Xransverse section of the upper 

 i^ J ^ ' part of the primary root of an older plant of 



by a few lignified cells, are collected into isolated PhaseoiusmHMjiorus^bbbbpnm^rypUoom- 



J o ' bundles; I?'* bast-fibres in the secondary phloem; 



groups. This occurs especially in the cultivated >& pericambium. 

 beet-root, the cultivated carrot, and in Althcea officinalis, Rheum rhaponticum, Atropa 

 Belladonna^ Cocculus palmatus. Inula Helenium, &c. The secondary phloem of roots 

 also has a tendency towards an abundant formation of parenchyma, with a diminished 

 development of bast. The secondary tissues of roots so completely resemble, on the 

 other hand, those of stems in the formation of xylem- and phloem-rays (medullary 

 rays) and in other respects, that it is not always possible, especially when the 

 wood of the root is not strongly lignified, to distinguish it from the wood of the 

 stem without examining the central axis, when its real nature may always be 

 determined from the primary xylem-bundles and the occasional absence of the 

 pith. 



From the great numbers of species belonging to the classes Gymnosperms and 

 Dicotyledons, and from their extremely different adaptations to various vital conditions, 

 it is not surprising that in the processes connected with the increase in size of the 

 stem there should be a great variety of deviations from the normal types described 



Very instructive figures of this are given by Van Tieghem, /. c. 



