Plants having little or no secondary growth are [2018]
grasses
deciduous angiosperms
conifers
cycads
(1)
Monocots (e.g., grasses) lack secondary growth, as they lack lateral meristem viz. vascular cambium and cork cambium. In case of conifers (e.g., Pinus) and cycads (e.g., Cycas) vascular tissues are arranged into vascular bundles just like angiosperms. They are open so secondary growth is common.
The vascular cambium normally gives rise to [2017]
primary phloem
secondary xylem
periderm
phelloderm
(2)
Cells of vascular cambium divide periclinally both on the outer and inner sides to form secondary permanent tissues, i.e., secondary xylem and secondary phloem.
Which of the following is made up of dead cells? [2017]
Collenchyma
Phellem
Phloem
Xylem parenchyma
(2)
The phellem or cork consists of dead and compactly arranged rectangular cells that possess suberised cell walls.
Identify the wrong statement in the context of heartwood. [2017]
It is highly durable.
It conducts water and minerals efficiently.
It comprises dead elements with highly lignified walls.
Organic compounds are deposited in it.
(2)
Heartwood is the non-functional part of secondary xylem, hence, it does not conduct water and minerals.
The balloon-shaped structures called tyloses [2016]
originate in the lumen of vessels
characterise the sapwood
are extensions of xylem parenchyma cells into vessels
are linked to the ascent of sap through xylem vessels
(3)
Tyloses are balloon-like extensions of parenchyma cells that protrude into the lumen of a neighbouring xylem vessel or tracheid through a pit in the cell wall. Tyloses form most commonly in older woody tissue, possibly in response to injury. They may eventually block the vessels and thus prevent the spread of fungi and other pathogens within the plant. Tyloses may become filled with tannins, gums, pigments, etc., giving heartwood its dark colour, and their walls can remain thin or become lignified.
Read the different components from (i) to (iv) in the list given below and tell the correct order of the components with reference to their arrangement from outer side to inner side in a woody dicot stem. [2015]
(i) Secondary cortex
(ii) Wood
(iii) Secondary phloem
(iv) Phellem
The correct order is:
(iv), (i), (iii), (ii)
(iv), (iii), (i), (ii)
(iii), (iv), (ii), (i)
(i), (ii), (iv), (iii)
(1)
In a woody dicot stem, phellem (cork) forms the outermost layer followed by phellogen and then secondary cortex (phelloderm). Secondary phloem forms a narrow circle on the outer side of vascular cambium, whereas secondary xylem occurs on the inner side of vascular cambium.
