In this article we will discuss about the secondary growth in dicot stem.
A. In the Intra-Stelar Region:
The secondary growth always begins in the intra-stelar region. The meristematic cells of the cambium of the vascular bundles, called fascicular cambium, begin to divide and produce new cells on the outer and inner sides. (Fig. 158)
Almost simultaneously a few parenchymatous cells of the medullary rays become meristematic in a line with the fascicular cambium of the vascular bundle and join with the latter to form a complete cambium ring.
Newly formed strips are known as inter-fascicular cambium. A cambium cell divides into two cells, one of which remains meristematic, and the other one is modified either into a xylem element or a phloem element.
That is how the cambium perpetuates itself. The newly formed cells on the inner side are modified into secondary xylem elements, and those on the outer side into secondary phloem elements. Thus the primary xylem and phloem are gradually pushed apart from each other.
The cambium cylinder always produces more xylem than phloem; so the ring, with all the tissues in front of it, is pushed more and more towards the periphery.
It has the same elements as in primary xylem, viz. vessels, tracheids, fibres and parenchyma. The formation of secondary xylem is not uniform throughout the year. Particularly in the places where seasonal variations are acute, some concentric rings are found in the hard woody part.
They are called annual rings. Each annual ring represents a year’s addition of secondary xylem, so by counting them the age of the plant may be determined.
Spring is the favourable season for plant growth, when a large number of new buds come out and unfold. The secondary xylem formed during spring is made up mainly of vessels with wide cavities. This wood is called spring wood or early wood. In autumn the rate of assimilation decreases, and, thus, the wood formed is mainly composed of hard lignified elements with smaller cavities.
This wood is called autumn wood or late wood. Winter is a period of rest when cambial activities come to a standstill, which is followed by spring and autumn of next year. Consequently a sharp line of demarcation exists between small-celled. In autumn wood of previous year and large-celled spring wood of current year. Spring and autumn wood together form an annual ring.
After considerable growth of the trees the central portion of the wood becomes very hard, durable and usually dark-coloured due to deposition of resin, tannins, etc. This hard central wood is called heart wood or duramen.
Heart wood does not conduct water but only serves as mechanical column. The outer light- coloured wood which, in fact, conducts water is called sap wood or alburnum. Though heart wood is more valuable from the commercial point of view, but biologically sap wood is more important.
It is made up of sieve tubes, companion cells, phloem parenchyma and fibres. Phloem shows no corresponding annual rings.
Some cambium cells produce thin-walled parenchyma which run as strips or channels in radial direction between the external and internal regions. They, in fact, belong to secondary phloem and secondary xylem, and are known as secondary rays or vascular rays.
B. In the Extra-Stelar Region:
Due to the formation of secondary tissues in the intra-stelar region considerable pressure is exerted on the epidermis which becomes stretched and tends to rupture. To prevent that a new meristem, called cork-cambium or phellogen, is produced on the superficial layers of cortex.
Cork-cambium cells divide as usual forming parenchymatous phelloderm or secondary cortex on the inner side and cork cells or phellem on the outer side. Cork cells are brick-shaped ones which lose protoplasm and their walls are suberised.
These suberised cork cells serve as secondary protective tissues. But as they are impervious to water, the cells lying outside the cork do not get any supply of food. They ultimately die and dry up to form what is known as bark.
So all the dead tissues lying outside the active cork-cambium constitute the bark. New phellogen may be formed in the deep-seated layers of cortex which also behaves in the same manner and produces quite thick bark. A continuous bark is called ring bark; but if the bark is formed in patches, as in guava, it is called scale bark.
After the formation of the secondary tissues the stomata present in the epidermis can no longer function. For carrying on gaseous interchange with the outer atmosphere new aerating pores, called lenticels, are formed. Lenticel originates just beneath a stoma.
The cork-cambium there, instead of producing suberised cork cells, gives rise to some loosely arranged parenchyma cells, called complementary cells, which rupture the epidermis and look like small swellings on the stem. The opening with the complementary cells is called the lenticel (Fig. 159).