Question

The secondary growth in stem of Cycas is brought about by:
A. Persistent cambium
B. Short lived cambium
C. Number of cambia produced by succession
D. Isolated strips of cambium

Answer 1

Hint: Plants exhibit two types of growth one is primary growth by which cell division occurs on the tips of roots and shoots and helps in their elongation and the secondary growth which occurs in the lateral meristem and leads to the thickening of the stem.

Complete answer:
Secondary growth occurs by cell division in the cambia or lateral meristem of the plants. Mainly two lateral meristems take part in the secondary growth of the plant – vascular and cork cambium. In secondary growth the lateral meristems keep on increasing the number of cells by cell division which results in the increase of width of the plant. This type of growth is responsible for the production of wood and thickened trunks of plants.
Cycas belongs to the class Cycadopsida and family Cycadaceae. Secondary growth in Cycas begins by the formation of secondary xylem and phloem outside the already present primary xylem and phloem and this happens by the activity of inter and intra - fascicular cambium which cuts the secondary xylem to the inner side and secondary phloem towards the outer side along with well transversed medullary rays. This way more than 15 rings can form by the secondary growth in the stem. Cambium rings that are present on the periphery of the stem have less vascular bundles. Same happens in the roots. So basically this development is initiated in the isolated strips of cambium.

Hence, the correct answer is option (D).

Additional information:
-Secondary growth takes place in all seed plants except monocots as the intra fascicular cambium is absent in them which are responsible for the initiation of secondary growth.
-Secondary growth is responsible for the ring formation in the plant which helps to calculate its age.
-Secondary xylem develops thin would during spring season and thick/dense wood during winters this is what marks a year of the plant’s age.

Note:
Cork cambium is the outermost lateral meristem that is present in the woody plants which contains a waxy substance called suberin which repels water and it has cork cells that makeup the bark of the tree. Bark prevents the tree from water loss and other physical damage and it mostly consists of dead cells.