Question

Tracheary elements of protoxylem are
(a) Larger and broader
(b) formed towards the end
(c) With the advanced type of thickenings
(d) Short narrow with a primitive type of thickenings

Answer 1

The tracheary elements of the protoxylem have annual or spiral thickenings in the secondary wall that they form and they are not of an advanced type. Tracheary elements are also found with a lumen which is not like that of a metaxylem that has large lumens.

Complete Step by Step Answer:
As you may know that protoxylems are formed at first as part of the xylem in vascular bundles. They tend to attain a mature state even when the other organs present in the plant body have not yet completely elongated hence the tracheary elements of the protoxylem too are formed at a much earlier stage of the life of the plant, hence the second option is not true. The protoxylem unlike the metaxylem undergo a lot of strain and stress in the stages of maturation of the plant, due to which the structure of the tracheids is thin and narrow, and having a pattern of spiral or annular thickenings. The thickenings too are not advanced as you may see in metaxylem rather they are of primitive type.

Additional Information:
- In the stem of plants, the protoxylem is arranged such that it faces the center while in the roots they are arranged towards the outside or the periphery.
- The protoxylem unlike that of metaxylem consists of much less amount of tracheary elements in it.
- Protoxylem also consists of comparatively much more amount of parenchyma than it is found in the metaxylem.
- In the monocot stems, there occurs a fusion of the protoxylems that leads to cavity formation which is also known as the lysigenous cavity.
- Protoxylem also shows the absence of fibers and also shows lesser efficiency in the mineral and water conduction.
- Protoxylem also shows the formation of tyloses which are protoplasmic structures.
So, the correct answer is ‘Short narrow with a primitive type of thickenings’.

Note:
- The formation of the lysigenous cavity is only shown in the vascular bundles of the monocot stems which hold water in the same.
- You must also note that these lysigenous cavities are present at the end of the vessels of the protoxylem and arise from the inner protoxylem parenchyma and vessels.