7. Histogen theory
• developed by Hanstein (1868, 1870)
• the main body of the plant arises from
the histogens
– Dermatogen -the precursor of the
epidermis
– Periblem -gives rise to the cortex
– Plerome- constitutes the inner mass
of the axis.
8. Histogen theory
• Assumes that the destinies of the
different regions of the plant body are
determined by the discrete origin of
these regions in the apical meristem.
9. Tunica- corpus organization
• Tunica layer shows anticlinal divisions
• Corpus –divides in various planes
• Exhibited by angiosperms
10. Cyto-histologic zonations
• Distal axial zone –terminating the axis
• Proximal axial zone (inner zone )-
appears directly below the distal zone;
usually becomes the pith
• Peripheral zone or outer zone (flank
meristem) –shows the smallest dimensions
and densest cytoplasm; most meristematic
• Transition zone –like a cambium; actively
dividing derivatives of the distal zone
11.
12.
13.
14. Cyto-histologic zonations
• Zonations based on:
– Planes of division
– Degree of meristematic activity
– Cytologic and histologic differentiation
• Shown in the apex of gymnosperms
19. Classification of meristems based
on the number of initials
1. one initial – many vascular cryptogams
e.g. Equisetum
2. several initials in one tier – most
gymnosperms; in Gnetales tunica-
corpus organization
3. several initials in more than one layer –
most angiosperms
20. Inactive center in the meristem
• Waiting meristem
• Quiescent center
-Ultimate source of all other cells
21. Origin of leaves
• Plastochron – time interval between two
successive similar events
• plastochronic changes in size of shoot
apex
-Minimal- area phase
-Maximal –area phase
22.
23.
24. Origin of branches
• formed in close association with leaves;
in the axils
• initiated later than the leaves subtending
them usually in the second plastochron
25.
26. Reproductive apex
• Modified shoot apex
• Changes in shoot apex:
– Nature of foliar organ subtending the bud changes
from foliar leaves to bracts
– the growth of the bud is earlier and more vigorous
than subtending bract
– Rapid increase in internodes in plants that are
rosette
27.
28.
29. Reproductive apex
• Changes in shoot apex:
– Shows uniform densely staining, small celled
mantle-like zone enclosing a lightly stained
core
-Disappearance of the rib meristem
-Indistinguishable plastochronic rhythms
30.
31. Root apex
• subterminal in position
• no branches initiated at the apex
-arise endogenously
• no lateral appendages comparable to
leaves
• do not undergo periodic changes in
structure
32. Types of root apical meristem
organization
single apical cell or from several initials in
one tier (vascular cryptogams)
open type- lacks clear differentiation of
initials
closed – characterized by the presence
of 3 tiers of initials
1st tier – apex of the central cylinder
2nd tier – terminates the cortex
3rd tier – gives rise to the rootcap
33.
34.
35. based on origin of epidermis
• rootcap and epidermis have common origin
– dermatocalyptrogen
• root cap has independent initials –
calyptrogen
• epidermis has its own initials –dermatogen
• cortex and epidermis have common initials
36. Functions of the cells in the
quiescent center
• 1. reservoir of cells resistant to damage
e g. radiation
• 2. site of auxin synthesis
• 3. source of diploid cells
• 4. permanent source of active initials
