Fig. 8.3-1. Longitudinal section of sieve tube members in stem of cucumber (Cucumis). This material was fixed in such a way as to enhance the formation of P-protein plugs, two of which are present here, stained red. When phloem is cut or chewed into, the release of pressure causes the P-protein to surge toward the sieve plate, where it becomes entangled. The plug is always on the side of the sieve plate opposite the damage: the first dissection cut in this sample -- the one that damaged the phloem -- was made above the top of the micrograph, not below the bottom. The plugs are widest at the sieve plate itself, and then are narrower farther away from the plate. Only the portion of the plug that is close to the plate has stained intensely.