Movement and redistribution of nutrients. Structure floemy

you are: Movement and redistribution of nutrients

Before to try to understand, how sucrose is transported on floeme, it is necessary for us to study structure sitovidnyh tubes. These thin-walled extended cages are connected by the ends form a continuous tube. In those places where adjacent cages adjoin with each other, their walls are penetrated numerous sitovidnymi by a time. Therefore trailer walls are called sitovidnymi as plates (fig. 8.3). Cages sitovidnyh tubes unlike cages ksilemnyh vessels live though they also are not similar to typical live cages. They have no kernel, but contain some other organelly and a plasmatic membrane (plazmalemmu), limiting external surface of a cage and sitovidnye a time. This plazmalemma plays the important role in deduction of sugars in sitovidnyh tubes; that cages sitovidnyh tubes can be plazmolizirovany testifies to its functional activity. Tonoplast, like a kernel and many other things organellam, no. By means of an electronic microentrenchment in sitovidnyh elements have been found out special albuminous mikrofilamenty, named F-fiber (floemnym by fiber). But the exact structure and function of these fibers are not known yet.

The Ultrastructure sitovidnyh tubes hardly gives in to studying as in the course of preparation of cuts the pressure existing in tubes, causes displacement of their contents by one end that leads to blocking of a time. For this reason representation was created that a time in a normal state is blocked. Proceeding from such representation, it was very difficult to understand, how a time carries out the transport function. However application of more thin methods in which floemu before cutting it is possible to fix for mikroskopirovanija in situ, has shown that contents sitovidnyh tubes usually do not block sitovidnuju a plate though in its time flows kallozy - amorphous polymer of glucose are occasionally found out. it has been established that at the majority of fast-growing plants such adjournment occur or as a result of damage of fabrics, or to normal conditions in the end of a functional life sitovidnyh tubes. In most cases sitovidnye tubes have the short period of a life and are constantly replaced new, formed at division kambija. Kalloza, possibly, functions in those plants which keep the sitovidnye tubes during a dormant period. At some trees, for example Tilia americana, sitovidnye a time is blocked by adjournment kallozy in the end of the growth period; by spring of a stopper kallozy are hydrolyzed, and hydrolysis products become a source of the substances used by a plant at renewal of the growth. As a result of destruction of stoppers it starts to be carried out again floemnyj transport of substances to places of their consumption.

Some parenhimnye the cages surrounding sitovidnye tubes, have special characteristics. They are formed together with sitovidnymi tubes at the final cell fission, forming sitovidnye tubes. Such parenhimnye cages are called as cages-companions. Cages-companions have the big kernel, dense cytoplasm, set of ribosomes and mitohondry, and also endoplazmatichesky retikulum. At transport of substances they can show special sekretornye functions. They, possibly, serve also as an energy source and information RNK, necessary for maintenance sitovidnyh tubes.