Delimitation of the protoplast in plant body due to presence of sharp wall is by no means complete. The primary wall is hardly uniform. Small breaks or meshes on the wall are traversed by extremely delicate and fine strands of cytoplasm to establish the connection between two adjacently-lying cells. These threads are called Plasmodesmata.
They are characteristic of all living cells. Plasmodesmata have been found in red algae, liverworts, mosses, pteridophytes, and spermatophytes.
They appear to remain aggregated in the primary pit fields. After formation of secondary walls they possibly remain restricted to the closing membranes of the pits.
On account of their extreme delicacy plasmodesmata cannot be normally demonstrated. With some special techniques, they may be readily seen in the endosperm of seeds with profuse stored food like date-palm – Phoenix and in the cotyledons. They are very thin in gymnosperms.
Though the presence of plasmodesmata definitely establishes the close relationship between the protoplast and the wall, their origin and development have been debatable.
Some workers believe that at the early stage of growth the wall is penetrated by cytoplasm, which becomes gradually narrower as a result of accumulation of microfibrils and pectic matters and ultimately form thin plasmodesmata threads.
Others are of opinion that plasmodesmata exist at the early cell-plate stage, and they also increase with the increase of the wall surface and often split.
Various suggestions have been put forward from time to time about their functions. Though nothing has been definitely known, it has been suggested that (i) they may be instrumental in conduction of stimuli – external and internal, through plant tissues; (ii) may be mainly concerned with translocation of food, particularly in the regions like endosperm; and (iii) may serve as channels for movement of some viruses.
Their occurrence in the haustoria-like bodies of parasites like Cuscuta and Orobanchi is suggestive of their roles in the movement of food and virus.