what are the differences between pits and plasmodesmata ?

Plasmodesmata are formed when portions of the endoplasmic reticulum are trapped across the middle lamella as new cell wall is laid down between two newly divided plant cells and these eventually become the cytoplasmic connections between cells (primary plasmodesmata). Here the wall is not thickened further, and depressions or thin areas known as pits are formed in the walls.

- Primary cell walls have thinner areas known as primary pit fields.

- Pit fields contain small openings in the wall through which cytoplasmic extensions known as plasmodesmata (singular, plasmodesma) extend between cells.

- The plasmodesmata are bounded by plasma membrane along their length, and a single tube of endoplasmic reticulum, the desmotubule extends through each plasmodesma.

- When the cell constructs a secondary cell wall, it doesn't lay down secondary wall over the primary wall's pit fields.

- This creates perforations in the secondary wall called pits. (Sometimes a pit is formed even if there's no pit field.)

- Pits of adjacent cells are usually appressed to each other so that the two primary cell wall and middle lamella form a selectively permeable pit membrane.
pit---primary wall---middle lamella---primary wall--pit
...comprises the pit pair through which the cells can transmit water, nutrients, hormones, etc.

Pits are found in the xylem.. the xylem has a lignified wall... however, there are regions on the cell wall that has no lignins....these regions are known as pits and water pass through pits to neighboring cells/tissues...

Plasmodesmata on the other hand can be thought of as pores/holes, watever on cell walls that allows materials to pass through across cells..a good example is vessel elements and companion cells....both cells exchange materials through the plasmodesmata....( I assume you are familiar with plant anatomy.).

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