Answer any two of the following: (a) Explain the system of classification in plants (b) Explain the secondary growth formation in dicotyledonous. Discuss the anomaloys growth in dicotyledonous stem using suitable example with labeled diagram. (c) Compare the male gametophytes of cycas, Pinus and Gnetum. (d) Describe the primary structure of monocote and dicote roots. Explain with suitable diagram, the secondary growth in dicot roots.
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Answer:
The growth that results from cell division in the cambia or lateral meristems which causes the stems and roots to thicken is called secondary growth
This type of growth occurs due to the activity of the two lateral meristems- the cork cambium and vascular cambium.
Secondary growth in dicots:-
1) Formation of cambium ring:
The cambium of vascualr bundles becomes meristematic.
Some of the medullary ray cells lying at the level of cambium also become meristematic.
It forms a strip of interfasciular cambium together with intrafasciular cambium to form a circular ring called cambium ring.
This cambium ring forms secondary tissues in the stellar region.
The vascular cambium consists of two types of cells- fusiform initials and ray initials.
The fusiform initials are vertically oriented which divide to form the elements of xylem and phloem.
2) Formation of secondary vascular tissues:
The cambium ring cuts off new cells, both on outer and inner sides,
The new cells formed on the outer side modify into the elements of secondary phloem.
The cells formed on the inner side modify into the secondary xylem.
3) Vascular rays:
Ray initials of the cambium ring form narrow bands of parenchymatous cells.
These cells extend radially from the pith to the phloem.
These are called secondary medullary rays or vascular rays.

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