. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. omesmore or less deeply cleft by thephloem (Fig. 30, A). In extremeinstances the wood may becomecut up into many isolated strandsby the production of secondarymeristems from cell division inthe wood parenchyma, medullaryrays and pith (Fig. 30, B), andby ingrowth of cells of the peri-cycle; and the secondary meri-stems may add new tissues tothese strands and even interpolatenew strands among them. The growth in thickness offleshy roots, tubers, and rhizomes,does not, as a rule, di
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. omesmore or less deeply cleft by thephloem (Fig. 30, A). In extremeinstances the wood may becomecut up into many isolated strandsby the production of secondarymeristems from cell division inthe wood parenchyma, medullaryrays and pith (Fig. 30, B), andby ingrowth of cells of the peri-cycle; and the secondary meri-stems may add new tissues tothese strands and even interpolatenew strands among them. The growth in thickness offleshy roots, tubers, and rhizomes,does not, as a rule, differ inmethod from the normal type, butthe cambium gives rise princi-pally to wood parenchyma andmedullary ray cells, or the cellsof the primary cortex and peri-cycle multiply enormously andconstitute most of the thickening;or, again, the secondary cortexmay be chiefly involved, as in theroot of Taraxacum omcinale. Anotable exception to the normaltype is found in the. root of thebeet where successive secondarycambium rings are formed outsidethe first and by their activity bringabout the thickening of the m FIG. 30.—Diagrams showing sometypes of unusual growth in , cross section through a four-year-oldstem of Anisostichus capreolata; B,cross section of stem of species ofBauhinia; the xylem strands, b, arestippled while the surrounding paren-chyma and bark tissues are left , portion of a cross section of stem ofGnetum scandens; i, 2 and 3 are success-ive rings of growth; m, is the pith; b, is asclerenchyma ring. The xylem portionswith the exception of the larger trachealtubes are shaded, while the medullaryrays, phloem and tissues interveningbetween the rings of growth and theouter cortex tissues are left white. (Aand C, after de Bary; B, after Schleiden. 60 SECONDARY INCREASE IN THICKNESS ILLUSTRATIVE STUDIES 1. Make cross and longitudinal sections of a stem of Aris-tolochia that is several years old (Fig. 24). The sectioning willneed to be done on a s
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