Question

I read in a textbook that the vascular cambium creates secondary xylem to its inside and outside. These cells, formerly conducting water, dry out and harden. Their cell walls get lignified creating wood, particularly in the cylinder between cork cambium and vascular cambium. This is actually three related questions. Is a force exerted on the secondary xylem by the vascular cambium? A force that compresses these cells?If yes to number one, how can soft tissues that make up the vascular cambium press so hard on the previous layers of harder (dried out) secondary xykem? Are concentric cylinders staggered so hardness increases gradually, with a stiffer already compressed cylinder exerting more force than newer secondary xylem cylinders?The secondary xylem (cortex) is living. Medullary rays traverse them and it is adjacent to the living cork cambium. As lignin is deposited in its cell walls, what biochemical processes continue to occur? The hard walls would deter intercellular transport but the cells must continue interacting.

Answer 1

The vascular cambium produces secondary xylem, which increases in hardness towards the center. The cambium exerts outward force, pushing new softer xylem layers outwards, thus forming concentric growth rings with varying density. Living cells in the xylem maintain limited intercellular transport despite lignification.

Step-by-step explanation:

The vascular cambium is responsible for the production of secondary xylem and phloem, playing a crucial role in the secondary growth of plants. Although the cambium is composed of softer tissues, the cells it produces exert a significant outward force as they expand. This expansion does not compress the secondary xylem inwards due to its lignified cell walls and attachment to adjacent cells but rather crushes cells to the outside, including the original epidermis.

The secondary xylem, which forms the bulk of wood in woody plants, is stiff and strong due to lignification. The wood's hardness increases towards the center of the stem as new layers of softer xylem are produced by the cambium and existing xylem is pushed outward. This creates concentric rings known as growth rings, which vary in density and represent early wood and later, denser late wood.

Regarding the biochemical processes in the secondary xylem, despite the presence of lignin, various cells like parenchyma and medullary rays remain alive and maintain intercellular interaction. These living cells allow for continued exchange and transport of substances, although the extent of this transport is limited by the presence of hardened cell walls.