49
Assume that a pencil box held in your hand, represents a plant cell. In how many possible planes can it be cut? Indicate these cuts with the help of line drawings.
Explanation
A. If a plant cell is cut in different plane if result, in radial symmetry.
B. If a plant cell is cut in two equal halves it result in bilateral symmetry.
50
Each of the following terms has some anatomical significance. What do these terms mean? Explain with the help of line diagrams.
(a) Plasmadesmoses/Plasmodesmata
(b) Middle lamella
(c) Secondary wall
Explanation
These terms mean as listed
| Structure | Function | Diagram |
|---|---|---|
| Plasmodesmata These are microscopic connecting channels between the two cells through the cell wall. |
Allow communication and transport between two neighbouring cells. Plasmodesmata allow molecules to travel between plant cells through the symplastic pathway. |
 |
| Middle lamella It is a layer in the cell wall mainly made of calcium pectate. |
Performs the function of cementing between the two neighbouring cells. |  |
| Secondary wall It is a non-extensible layer made of hemicellulose fibres, in the cell wall of plant cells. |
Provides rigidity to the cell wall in plant cell. |  |
51
Distinguish between the following.
(a) Exarch and endarch condition of protoxylem
(b) Stele and vascular bundle
(c) Protoxylem and metaxylem
(d) Interfascicular cambium and intrafascicular cambium
(e) Open and closed vascular bundles
(f) Stem hair and root hair
Explanation
Differences between the following
| (a) | Exarch Protoxylem | Endarch Protoxylem |
|---|---|---|
| When the protoxylem is present towards the periphery and metaxylem towards centre in vascular bundle as it is found in roots. | If the protoxylem is present towards the centre and metaxylem towards periphery in the vascular bundle as it is found in stem. | |
| (b) | Stele | Vascular Bundle |
Stele refers to conducting tissue or the central part of root or stem in plants. It comprises of vascular tissue, ground tisssue and pith and limiting boundaries, i.e., endodermis and pericycle. |
Vascular bundle comprises of vascular/conducting tissues xylem and phloem. Some times cambium is also included as in dicots. |
|
| (c) | Protoxylem | Metaxylem |
It is the first or earlier formed xylem. Matures before the gorwth and differentiation of plant organs. Protoxylem elements are smaller in diameter. Tyloses absent in protoxylem vessels. Fibres are absent. |
It is the later formed xylem. Matures after the growth and differentiation of plant organs. Metaxylem delements are broader and greater in diameter. Tyloses are generally present. Fibres may be present. |
|
| (d) | Intrafascicular Cambium | Inter fascicular Cambium |
| Cambium present in between the primary xylem and primary phloem is called-intra fascicular cambium, as in dicot stems. |
The cambium present in between the two vascular bundles making the cambium continuous and forming a complete ring of cambium is called interfascicular conbium. | |
 |
||
| (e) | Open Vascular Bundle | Closed Vascular Buncle |
| Cambium present in between the xylem and phloem tissue. Intrafascicular cambium between the phloem present. Occur in the stems of dicot and gymnosperms. May be collateral or bicolateral. Xylem and phloem not in direct contact with each other due to cambial string. Intrafascicular cambium results in secondary growth. Cambial activity produces secondary phloem and secondary xylem that push primary phloem and primary xylem away from each other.  |
Cambium is not present in between the xylem and phloem tissue. Intrafascicular cambium absent. Occur in leaves and monocot stems. May be collateral or concentric. Xylem and phloem are in direct contact due to back of cambial string. No such activity found. No such activity found.  |
|
| (f) | Root Hair | Stem Hair |
Root hairs are unicellular  It increase the surface area of root for
absorption of water and minerals. |
Stem hairs are multicellular
They are epidermal outgrowths known as trichomes. They help in preventing water loss due to transpiration. |
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