BOTANY Higher Secondary Second Year - Textbooks Online

If xylem and phloem in a vascular bundle are arranged along the
same radius with phloem towards the outside, such vascular bundle is
called collateral vascular bundle. If phloem occurs on both the outer and
inner sides of xylem, the bundle is called bicollateral. Bicollateral vascular
bundles are most typically seen in Cucurbitaceae.
The bundle in which either phloem surrounds the xylem or xylem
surrounds the phloem completely is known as concentric vascular bundle.
This is of two types amphicribral and amphivasal. In amphicribral
concentric vascular bundles, the phloem completely surrounds the xylem.
eg. Polypodium. In amphivasal concentric vascular bundles, the xylem
completely surrounds the phloem. eg. Acorus. In roots, protoxylem vessels
are present towards the periphery and the metaxylem vessels towards the
centre. This arrangement of xylem is called exarch. In stem, protoxylem
vessels are towards the centre, while metaxylem towards the periphery.
This condition is known as endarch.
Ground or fundamental tissue system
The ground or fundamental tissue system constitutes the main body of
the plants. It includes all the tissues except epidermis and vascular bundles.
In monocot stem, ground tissue system is a continuous mass of
parenchymatous tissue in which vascular bundles are found scattered.
Here ground tissue is not differentiated into cortex, endodermis, pericycle
and pith. Generally in dicot stem, ground tissue system is differentiated
into three main zones - cortex, pericycle and pith.
The cortex occurs between the epidermis and pericycle. Cortex may
be a few to many layers in thickness. In most cases, cortex is made up of
parenchyma tissues. Intercellular spaces may or may not be present..
Cortical cells may contain non-living
inclusions like starch grains, oils,
tannins and crystal.
Casparian strip
Fig. 2.10. Endodermal cells
with casparian strips
In the leaves, the ground tissue
consists of chlorenchyma tissues. This
region is called mesophyll. The inner
most layer of the cortex is called
endodermis. Generally endodermis is
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