SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 
Precisely similar to the growth in thickness in Phaseolns is that of the primary 
roots containing pith of seedlings of Cucurbita Pepo, 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 Tropccolum, but project much further outwardly. When there are two 
of these primary xylem-bundles, as in Beta, Tj'opcEolwn, 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, Pinns, 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 Fif^- loy.-Transverse section of the upper 
J ' ' part of the primary root of an older plant of 
by a few lisrnified cells, are collected into isolated Phaseoius jnuitißorus ; b h t b primary ph^^^^^ 
•' o ' bundles; 5 bast-fibres m the secondary phloem; 
groups. This occurs especially in the cultivated pencambium. 
beet-root, the cultivated carrot, and in Althcea officinalis, Rheum rhaponticum, Atropa 
Belladonna^ Cocculus palinatus. 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, I.e. 
