SECONDARY GROWTH IN STEM
The stem increases in thickness In sccondary growth. Secondary growth increases the Vascular tissues. Therefore, it increases the thickness of flue stem. Secondary Growth is common in Gymnosperms and woody dicots. But it is Generally absent in most of the herbaceous dicots and monocots Secondary growth includes the formation of secondary Vascular tissues and periderm.
The faseicular cambium is present between xy lem and phloem tissues.The vascular cambium of the bundles is not Continuous With
adjacent bundles. The Parenchyma cells of the medullary rays are present in between the fascicular cambium. The parenchyma cells of medullary rays become Meristematic. They form strip of interfascicular cambium in medullary rays. The strips of fascicular
and interfascicular cambium join together laterally to form a continuous cambium ring called vascular cambium. This vascular cambium form secondary xy lem and phloem.
The cambial cells have different morphology from the meristematic cells of the apical meristem. The cambial cells are highly vacuolated. They are of Two forms.
(a) Fusiform initials: They are wedge like. They are many times
longer than broad and have narrow pointed ends. Therefore. these cells are rectangular in transverse section. The derivatives of the fusiform initials towards the inner side differentiate into secondary xylem and those towards outer side differentiate into secondary phloem.
(b). Ray initials: They are nearly isodiametric. They are found in vertical Rows. The Derivatives of Ray initials give rise to rays.
Secondary Vascular Tissues
(a) Secondary xylem
Secondary xylems are produced Towards the inner side. It pushes the primary xylem bundles towards the centre. Ultimately, the pith is completely filled With the secondary xy lem and primary’ xy lent elements are crushed. The secondary xylem in Dicots consists of an axial or vertical system and rays
(a) Axial or vertical systems: The xylems have vessels. some tracheids, fibres and axial parenchyma
(1) Ray system: The horizontal or ran system consists of files of parenchyma cells. They Run radially along the Nt hole length ol secondary xylem and secondary phlown Hie ray may be
imiseriate or multi:seri:Ile. I niseriate is -ingle cell ide.
Multiseriate is many celled sy ide. ilie margins of multiseriate rays are Assays one cell k%kle. NO\ l’anS are continuously produced in the second:11y xy lent and phloem.
Formation of wood
Ihe secondary xylem accumulates. It forms \sou& Wood has Rs o parts:
(a) Sap wood: –I he ‘Limns peripheral Functional part of the
second:if:. !cm is :ailed Nap ss ooT
(b) Heart %vocal : major central nonfunctional part form, the
lteart ood. I :IC LiLttlUl ii heart )..t),,Li underv certain
mot pho;didic, cliane.i lics2 clianp2, hICILICIC deposition of
tannin., and ! C;otnetitne-:. the adjacent